Kershaw was born into a working-class, Catholic family in Oldham and was educated at the local grammar school During his youth, he became fascinated with the early modern and medieval periods when England was Catholic. Educated at St Bede's College, Manchester, the University of Liverpool (BA) and Merton College, Oxford (D.Phil), Kershaw was originally trained as a medievalist but turned to the study of modern German social history in the 1970s. At first, Kershaw was mainly concerned with the economic history of Bolton Abbey. As a Lecturer in Medieval History at Manchester, Kershaw learned German to study the German peasantry in the Middle Ages. In 1972, Kershaw visited Bavaria and was shocked to hear the views of an old man he met in a Munich cafe who told him: "You English were so foolish. If only you had sided with us. Together we could have defeated Bolshevism and ruled the earth!", adding in for good measure that "The Jew is a louse!". Kershaw became keen to learn how and why ordinary people in Germany could support National Socialism as a result of this incident.

In 1975, Kershaw joined Martin Broszat's "Bavaria Project". During his work, Broszat encouraged Kershaw to examine how ordinary people viewed Hitler. As a result of his work in the 1970s on Broszat's "Bavaria Project" , Kershaw wrote his first book on the Third Reich, The 'Hitler Myth': Image and Reality in the Third Reich which was first published in German in 1980 as Der Hitler-Mythos: Volksmeinung und Propaganda im Dritten Reich. This book examined the "Hitler cult" in Germany, how it was developed by Joseph Goebbels, what social groups the Hitler Myth appealed to and how it rose and fell.

Also arising from the "Bavaria Project" and Kershaw's work in the field of Alltagsgeschichte was Popular Opinion and Political Dissent in the Third Reich. In this 1983 book, Kershaw examined the experience of the Third Reich at the grass-roots in Bavaria. Kershaw showed how ordinary people reacted to the Nazi dictatorship, looking at how people conformed to the regime and to the extent and limits of dissent. Kershaw described his subject as ordinary Bavarians, or as he referred to:

"the muddled majority, neither full-hearted Nazis nor outright opponents, whose attitudes at one and the same time betray signs of Nazi ideological penetratation and yet show the clear limits of propaganda manipulation".

Kershaw went on to write in his preface:

"I should like to think that had I been around at the time I would have been a convinced anti-Nazi engaged in the underground resistance fight. However, I know really that I would have been as confused and felt as helpless as most of the people I am writing about".

Kershaw argued that Goebbels failed to create the Volksgemeinschaft (people's community) of Nazi propaganda, and that most Bavarians were far more interested in their day to day lives than in politics during the Third Reich. Kershaw concluded that the majority of Bavarians were either anti-Semitic or more commonly simply did not care about what was happening to the Jews. Kershaw also concluded that there was a fundamental difference between the anti-Semitism of the majority of ordinary people, who disliked Jews and were much colored by traditional Catholic prejudices, and the ideological and far more radical völkische anti-Semitism of the Nazi Party, who hated Jews. Kershaw found that the majority of Bavarians disapproved of the violence of Kristallnacht pogrom, and that despite the efforts of the Nazis, continued to maintain social relations with the members of the Bavarian Jewish community. Kershaw documented numerous campaigns on the part of the Nazi Party to increase anti-Semitic hatred, and noted that the overwhelming majority of anti-Semitic activities in Bavaria were the work of a small number of committed Nazi Party members. Overall, Kershaw noted that the popular mood towards Jews was indifference to their fate. Kershaw argued that during World War II, most Bavarians were vaguely aware of the Holocaust, but were vastly more concerned about and interested in the war than about the "Final Solution to the Jewish Question". Kershaw made the notable claim that:

"the road to Auschwitz was built by hate, but paved with indifference"..

By this, Kershaw meant the progress leading up to Auschwitz was motivated by anti-Semitism of the most vicious kind held by the Nazi elite, but it took place in a context where the majority of German public opinion was completely indifferent to what was happening.

The British comedian Sacha Baron Cohen has stated that Kershaw’s statement about “indifference” as the predominant public reaction to the Holocaust inspired in part his Borat Sagdiyev character. Cohen stated that:

"When I was in university, there was this major historian of the Third Reich, Ian Kershaw, who said, 'The path to Auschwitz was paved with indifference.' I know it's not very funny being a comedian talking about the Holocaust, but it's an interesting idea that not everyone in Germany had to be a raving anti-Semite. They just had to be apathetic".

Kershaw’s assessment that most Bavarians, and by implication Germans were “indifferent” to the Shoah faced criticism from the Israeli historian Otto Dov Kulka and the Canadian historian Michael Kater. Kater contended that Kershaw downplayed the extent of popular anti-Semitism, and that though admitting that most of the “spontaneous” anti-Semitic actions of Nazi Germany were staged, argued that because these actions involved substantial numbers of Germans, it is wrong to see the extreme anti-Semitism of the Nazis as coming solely from above. Kulka argued that most Germans were more anti-Semitic than Kershaw portrayed them in Popular Opinion and Political Dissent in the Third Reich, and that rather than “indifference” argued that “passive complicity” would be a better term to describe the reaction of the German people to the Shoah.

In 1985, Kershaw published a book on the historiography of Nazi Germany entitled The Nazi Dictatorship, in which Kershaw reflected on the problems in historiography of the Nazi era. Kershaw noted the huge disparity of often incompatible views about the Third Reich such as the debate between those who see the Nazi period as the culmination of Deutschtum (Germanism) and Marxists who see National Socialism as the culmination of capitalism; between those who argue for a Sonderweg and those who argue against the Sonderweg concept; between those who see National Socialism as a type of totalitarianism and those who see it as a type of fascism and between those historians who favour a “functionalist” interpretation with the emphasis on the German bureaucracy and the Holocaust as an ad hoc process and those who favour an “intentionist” interpretation with the focus on Hitler and the argument that the Holocaust had been something planned from early on in Hitler's political career. As Kershaw noted, these divergent interpretations such as the differences between the functionalist view of the Holocaust as caused by a process and the intentionist view of the Holocaust as caused by a plan are not easily reconciled, and that there was in his opinion the need for a guide to explain the complex historiography surrounding these issues. Likewise, if one accepts the Marxist view of National Socialism as the culmination of capitalism, then the Nazi phenomenon is universal, and fascism can come to power in any society where capitalism is the dominant economic system, whereas the view of National Socialism as the culmination of Deutschum means that the Nazi phenomenon is local and particular only to Germany. For Kershaw, any historian writing about the period had to take account of the "historical-philosophical", "political-ideological" and moral problems associated with the period, which thus poses special challenges for the historian. In The Nazi Dictatorship, Kershaw surveyed the historical literature, and offered his own assessment of the pros and cons of the various approaches. In a 2008 interview, Kershaw lists as his major intellectual influences Martin Broszat, Hans Mommsen, Alan Milward, Timothy Mason, Hans-Ulrich Wehler, William Carr and Jeremy Noakes. In the same interview, Kershaw expressed strong approval of Mason's "Primacy of Politics" concept against the orthodox Marxist "Primacy of Economics" concept, in which it was German Big Business who served the Nazi regime rather than the other way around. Despite his praise and admiration for Mason, in the 2000 edition of The Nazi Dictatorship, Kershaw was highly sceptical of Mason's "Flight into War" theory of an economic crisis in 1939 forcing the Nazi regime into war.

In the Historikerstreit (Historians' Dispute) of 1986 to 1989, Kershaw followed Broszat in criticizing the work and views of Ernst Nolte, Andreas Hillgruber, Michael Stürmer, Joachim Fest and Klaus Hildebrand, all of whom Kershaw saw as German apologists attempting to white-wash the German past in various ways. In the 1989 edition of The Nazi Dictatorship, Kershaw devoted an entire chapter towards rebutting the views of Nolte, Hillgruber, Fest, Hildebrand and Stürmer. In regard to the debate between those who regard National Socialism as a type of totalitarianism (and thus having more in common with the Soviet Union) versus those who regard Nazism as a type of fascism (and thus having more in common with Fascist Italy), Kershaw, though feeling that the totalitarianism approach is not without value, has argued that in essence, Nazism should be viewed as a type of fascism, albeit fascism of a very radical type. Writing of the Sonderweg debate, Kershaw finds the moderate Sonderweg approach of Jürgen Kocka the most satisfactory historical explanation for why the Third Reich occurred. In the 2000 edition of The Nazi Dictatorship, Kershaw wrote he considered Gerhard Ritter's claim that one “madman” (i.e. Hitler) single-handedy caused World War II to that of a German apologist, and that he found the historical approach of Ritter’s arch-enemy Fritz Fischer to be a far better way of understanding German history. Along the same lines, Kershaw criticized as German apologetics the 1946 statement by the German historian Friedrich Meinecke that National Socialism was just a particularly unfortunate Betriebsunfall (industrial accident) of history. Kershaw was later in a 2003 essay to criticize both Ritter and Meinecke as German apologists who either through the Betriebsunfall theory and by blaming everything upon Hitler were seeking to white-wash the German past Writing of the work of the German historian Rainer Zitelmann, Kershaw has argued that Zitelmann has elevated what were merely secondary considerations in Hitler's remarks to the primary level, and that Zitelmann has not offered a clear definition of what he means by "modernization".

In regards to the Nazi foreign policy debate between “globalists” such as Klaus Hildebrand, Andreas Hillgruber, Jochen Thies, Gunter Moltman and Gerhard Weinberg, who argue that Germany aimed at world conquest, and the "continentalists” such as Hugh Trevor-Roper, Eberhard Jäckel, and Axel Kuhn, who argue that Germany aimed only at the conquest of Europe,, Kershaw tends towards the “continentalist” position. Kershaw agrees with the thesis that Hitler did formulate a programme for foreign policy centring around an alliance with Britain to achieve the destruction of the Soviet Union, but has argued that a British lack in interest doomed the project, thus leading to the situation in 1939, where Hitler went to war with Britain, the country he wanted as an ally, as an enemy, and the country he wanted as an enemy, the Soviet Union, as his ally. At the same time, Kershaw sees considerable merit in the work of such historians as Timothy Mason, Hans Mommsen, Martin Broszat and Wolfgang Schieder, who argue that Hitler had no “programme” in foreign policy, and instead contend that his foreign policy was simply a kneejerk reaction to domestic pressures in the economy and his need to maintain his popularity.

In regards to the historical debates about Widerstand (resistance) in German society, Kershaw has argued that there are two approaches to the question, one of which he calls the fundamentalist (dealing with those committed to overthrowing the Nazi regime) and the societal (dealing with forms of dissent in "everyday life"). In Kershaw's viewpoint, Broszat's Resistenz (immunity) concept works well in an Alltagsgeschichte approach, but works less well in the field of high politics, and moreover by focusing only on the "effect" of one's actions, fails to consider the crucial element of the "intention" behind one's actions. Kershaw has argued that the term Widerstand should be used only for those working for the total overthrow of the Nazi system, and those engaging in behavior which was counter to the regime's wishes without seeking to overthrow the regime should be included under the terms opposition and dissent, depending upon their motives and actions. In Kershaw's opinion, there were three bands ranging from dissent to opposition to resistance. Kershaw has used the Edelweiss Pirates as an example of whose behavior initially fell under dissent, and who advanced from there to opposition and finally to resistance. In Kershaw's view, there was much dissent and opposition within German society, but outside of the working-class, very little resistance. Though Kershaw has argued that the Resistenz concept has much merit, he concluded that the Nazi regime had a broad basis of support and it is correct to speak of "resistance without the people".

Regarding the debate in the late 1980s between Martin Broszat and Saul Friedländer over Broszat's call for the "historicization" of National Socialism, Kershaw wrote that he agreed with Friedländer that the Nazi period could not be treated as a "normal" period of history, but he felt that historians should approach the Nazi period as they would any other period of history In support of Broszat, Kershaw wrote that an Alltagsgeschichte approach to German history, provided that it did not lose sight of Nazi crimes, had much to offer as a way of understanding how those crimes occurred.

During the "Goldhagen Controversy" of 1996, Kershaw took the view that his friend, Hans Mommsen had "destroyed" Daniel Goldhagen's arguments about a culture of "eliminationist antisemitism" in Germany during their frequent debates on German TV Kershaw wrote that he agreed with Eberhard Jäckel's assessment that Hitler's Willing Executioners was "simply a bad book" Through Kershaw had little positive to say about Goldhagen, he wrote that he felt that Norman Finkelstein had attacked Goldhagen in an over the top manner that did little to help historical understanding However, Kershaw later went on to recommend Norman Finkelstein and Rush Bettina Birn's extremely critical assessment of Goldhagen's book, A Nation On Trial; stating that "Finkelstein and Birn provide a devastating critique of Daniel Goldhagen's simplistic and misleading interpretation of the Holocaust. Their contribution to the debate is, in my view, indispensable."

Like Broszat, Kershaw sees the structures of the Nazi state as far more important than the personality of Hitler (or any other individual for that matter) as explanation for the way Nazi Germany developed. In particular, Kershaw subscribes to the view argued by Broszat and the German historian Hans Mommsen that Nazi Germany was a chaotic collection of rival bureaucracies in perpetual power struggles with each other. In Kershaw's view, the Nazi dictatorship was not a totalitarian monolith, but rather comprised an unstable coalition of several blocs in a "power cartel" comprising the NSDAP, big business, the German state bureaucracy, the Army and SS/police agencies (and moreover, each of the “power blocs” in turn were divided into several factions). In Kershaw's opinion, the more "radical" blocs such as the SS/police and the Nazi Party gained increasing ascendency over the other blocs after the 1936 economic crisis, and then onwards increased their power at the expense of the other blocs.

Kershaw, the real significance of Hitler lies not in the dictator himself, but rather in the German people's perception of him. In his biography of Hitler, Kershaw presented him as the ultimate “unperson”; a boring, pedestrian man devoid of even the “negative greatness” attributed to him by Joachim Fest. Kershaw has no time for the Great Man theory of history and has criticised those who seek to explain everything that happened in the Third Reich as the result of Hitler’s will and intentions. Kershaw has argued that it is absurd to seek to explain German history in the Nazi era solely through Hitler as Germany had sixty-eight million people during the Third Reich, and to seek to explain the fate of sixty-eight million people solely though the prism of one man is in Kershaw’s opinion a flawed position. Kershaw wrote about the problems of an excessive focus on Hitler that "...even the best biographies have seemed at times in danger of elevating Hitler's personal power to a level where the history of Germany between 1933 and 1945 becomes reduced to little more than an expression of the dictator's will". Kershaw has a low opinion of those who seek to provide "personalized" theories about the Holocaust and/or World War II as due to some defect, medical or otherwise in Hitler. In his 2000 edition of The Nazi Dictatorship, Kershaw quoted with approval the dismissive remarks made by the German historian Hans-Ulrich Wehler in 1980 about such theories. Wehler wrote:

"Does our understanding of National Socialist policies really depend on whether Hitler had only one testicle?...Perhaps the Führer had three, which made things difficult for him, who knows?...Even if Hitler could be regarded irrefutably as a sado-masochist, which scientific interest does that further?...Does the "Final Solution of the Jewish Question" thus become more easily understandable or the "twisted road to Auschwitz" become the one-way street of a psychopath in power?".

Kershaw shares Wehler's opinion, that for besides for the problem that such theories about Hitler's medical condition were extremely difficult to prove, was that they had the effect of personalizing the phenomena of Nazi Germany by more or less attributing everything that happened in the Third Reich to one flawed individual.

Kershaw’s biography of Hitler is an examination of Hitler’s power; how he obtained it and how he maintained it. Following up on ideas that he had first introduced in a 1991 book about Hitler, Kershaw has argued that Hitler's leadership is a model example of Max Weber's theory of Charismatic leadership. Kershaw's 1991 book Hitler A Profile In Power marked a change for Kershaw from writing about how people viewed Hitler to about Hitler himself In his two-volume biography of Hitler published in 1998 and 2000, Kershaw stated "What I tried to do was to embed Hitler into the social and political context that I had already studied" Kershaw finds the picture of Hitler as a “mountebank” (opportunistic adventurer) in Alan Bullock's biography unsatisfactory, and Joachim Fest's quest to determinate how "great" Hitler was senseless In a wider sense, Kershaw sees the Nazi regime as part of a broader crisis which afflicted European society from 1914 to 1945. Through in disagreement with many of their claims (especially Nolte’s), Kershaw’s concept of a “Second Thirty’s Years’ War” reflects many similarities with Ernst Nolte, A. J. P. Taylor and Arno J. Mayer who have also advanced the concept of a “Thirty Years’ Crisis” to explain European history between 1914-1945. .

In the Functionalism versus intentionalism debate, Kershaw has argued for a synthesis of the two schools, though Kershaw leans towards the functionalist school. Despite some disagreements, Kerhaw has called Mommsen a “good personal friend” and an “important further vital stimulus to my own work on Nazism".. Kershaw has argued in his two-volume biography of Hitler that Hitler did play a decisive role in the development of policies of genocide, but also argued that many of the measures that led to the Holocaust were undertaken by many lower-ranking officials without direct orders from Hitler in the expectation that such steps would win them favour. Though Kershaw does not deny the radical anti-Semitism of the Nazis, he favors Mommsen’s view of the Holocaust being caused by the “culminative radicalization” of the Third Reich caused by the endless bureaucratic power struggles and a turn towards increasingly radical anti-Semitism within the Nazi elite. Despite his background in the functionalist historiography, Kershaw admits that his account of Hitler in World War II owes much to intentionalist historians like Gerhard Weinberg, Hugh Trevor-Roper, Lucy Dawidowicz and Eberhard Jäckel Kershaw accepts the picture of Hitler drawn by intentionalist historians as a fanatical ideologue who was obsessed with Social Darwinism, völkisch anti-Semitism (in which the Jewish people were viewed as a "race" biologically different from the rest of humanity rather than a religion), militarism and the perceived need for Lebensraum However, in a 1992 essay, "'Improvised genocide?'", in which Kershaw traces how the ethnic cleansing campaign of Gauleiter Arthur Greiser in the Wargenthau region annexed to Germany from Poland in 1939 led to a campaign of genocide by 1941, Kershaw argued that the process was indeed "improvised genocide" rather the fulfilment of a masterplan Kershaw's views the Holocaust not as a plan as argued by the intentionists, but rather a process caused by the “culminative radicalization” of the Nazi state as articulated by the functionalists. Citing the work of the American historian Christopher Browning in his biography of Hitler, Kershaw argues that in the period 1939-41 the phrase "Final Solution to the Jewish Question" was a "territorial solution", that such plans as the Nisko Plan and Madagascar Plan were serious and only in the latter half of 1941 did the phrase "Final Solution" come to refer to genocide. This view of the Holocaust as a process rather than a plan is the antithesis of the extreme intentionist approach as advocated by Lucy Dawidowicz, who argue that Hitler had decided upon genocide as early as November 1918, and that everything he did from that time onwards was directed towards that goal.

In recent years Kershaw has come to form a thesis based on the ideas of both traditions of Nazi theory.

Kershaw disagrees with Mommsen's "Weak Dictator" thesis: the idea that Hitler was a relatively unimportant player in the Third Reich. However, he has agreed with his idea that Hitler did not play much of a role in the day-to-day administration of Nazi Germany. Kershaw's way of explaining this paradox is his theory of "Working Towards the Führer", the phrase being taken from a 1934 speech by the Prussian civil servant Werner Willikens. Kershaw has argued that in Nazi Germany, officials of both the German state and Party bureaucracy usually took the initiative in beginning policy to meet Hitler's perceived wishes, or alternatively attempted to turn into policy Hitler’s often loosely and indistinctly phrased wishes. Though Kershaw does agree that Hitler possessed the powers that the "Master of the Third Reich" thesis championed by Norman Rich and Karl Dietrich Bracher would suggest, Kershaw has argued that Hitler was a "Lazy Dictator"; an indifferent dictator who really did not have the interest to involve himself much in the daily running of Nazi Germany. The only exceptions were the areas of foreign policy and military decisions, both areas that Hitler increasingly involved himself in from the late 1930s.

In a 1993 essay entitled "'Working Towards the Führer'", Kershaw argued that the German and Soviet dictatorships had more differences then similarities. Kershaw argued that Hitler was a very unbureaucratic leader who was highly adverse to paper work in marked contrast to Stalin. Likewise, Kershaw argued that Stalin was highly involved in the running of the Soviet Union in contrast to Hitler whose involvement in the day to day decision making was limited, infrequent and capricious. Kershaw argued that the Soviet regime, despite all of its extreme brutality and utter ruthlessness was basically rational in its goal of seeking to modernize a backward country and had no equivalent of the "cumulative radicalization" towards increasing irrational goals that Kershaw sees as marking Nazi Germany. In Kershaw's opinion, Stalin's power corresponded to Weber's category of bureaucratic authority whereas Hitler's power corresponded to Weber's category of charismatic authority In Kershaw's view, what happened in Germany after 1933 was the imposition of Hitler's charismatic authority on top of the "legal-rational" authority system that had existed prior to 1933, leading to a gradual breakdown of any system of ordered authority in Germany. Kershaw argues that by 1938, the German state had been reduced to a hopeless, polycratic shambles of rival agencies all competing with each other to win Hitler's favor, which by that time had become the only source of political legitimacy. Kershaw sees this rivalry as causing the "cumulative radicalization" of Germany, and argues that though Hitler always favored the most radical solution to any problem, it was German officials themselves in attempting to win the Führer's approval who for the most part carried out on their own initiative increasingly "radical" solutions to perceived problems like the "Jewish Question" as opposed to being ordered to do so by Hitler. In this, Kershaw largely agrees with Mommsen's portrait of Hitler as a distant and remote leader standing in many ways above his own system, whose charisma and ideas served to set the general tone of politics. As an example of how Hitler's power functioned in practice, Kershaw used Hitler's directive to the Gauleiters Albert Forster and Arthur Greiser to "Germanize" the part of north-western Poland annexed to Germany in 1939 within the next 10 years with his promise that "no questions would be asked" about how this would be done. As Kershaw notes, the completely different ways Forster and Greiser sought to "Germanize" their Gaues with Forster simply having the local Polish population in his Gau signing forms saying they had "German blood" and Greiser carrying out a program of brutal ethnic cleansing of Poles in his Gau showed both how Hitler set events in motion, and how his Gauleiters could carry out totally different policies in pursuit of what they believed to be Hitler's wishes. In Kershaw's opinion, Hitler's vision of a racially cleansed Volksgemeinschaft provided the impetus for German officials to carry out increasing extreme measures to win his approval, and which ended with the Shoah.

The Israeli historian Otto Dov Kulka has praised the concept of “working towards the Führer” as the best way of understanding how the Holocaust occurred, combining the best features, and avoiding the weaknesses of both the “functionalist” and “intentionalist” methods. Kulka argued that Kershaw demonstrated both Hitler’s central role in the “Final Solution” and why there was no need for any order from Hitler for the Holocaust, as the progress that led to the Shoah were “worked out” toward the Führer by almost everyone in Germany.

Thus, for Kershaw Nazi Germany was both a monocracy (rule of one) and polycracy (rule of many). Hitler held absolute power but did not choose to exercise it very much; the rival fiefdoms of the Nazi state fought each other and attempted to carry out Hitler's vaguely worded wishes and dimly defined orders by "Working Towards the Führer".