Born in Tientsin, China, to missionaries Roscoe and Grace Baird Hersey, John Hersey learned to speak Chinese before he spoke English. He returned to the United States with his family when he was ten years old. Later Hersey attended the Hotchkiss School, followed by Yale University, where he was a member of Skull and Bones society. He subsequently was a graduate student at Cambridge as a Mellon Fellow. Following his time at Cambridge, Hersey got a summer job as private secretary and driver for author Sinclair Lewis in 1937, but he chafed at his duties, and that fall he began work at Time, where he was hired after writing an essay on the magazine's dismal quality. Two years later he was transferred to Time's Chongqing bureau.

During World War II, newsweekly correspondent Hersey covered fighting in Europe as well as Asia, writing articles for Time as well as Life magazine. He accompanied Allied troops on their invasion of Sicily, survived four airplane crashes, and was commended by the Secretary of the Navy for his role in helping evacuate wounded soldiers from Guadalcanal.

At the close of the conflict, during the winter of 1945—46, Hersey was in Japan, reporting for The New Yorker on the reconstruction of the devastated country, when he stumbled across a document written by a Jesuit missionary who had survived the atomic bomb dropped on Hiroshima. The journalist paid a call on the missionary, who introduced him to other survivors.

Shortly afterwards John Hersey began discussions with William Shawn, an editor at The New Yorker, about a lengthy piece about the previous summer's bombing. Hersey proposed a story that would convey the cataclysmic narrative through six individuals who survived: the Jesuit priest; a widowed seamstress; two doctors; a minister; and a young woman who worked in a factory. The following May, 1946, Hersey traveled to Japan, where he spent three weeks doing research and interviewing survivors. He returned to America in late June and began writing.

The result was his most notable work, the 31,000-word article "Hiroshima", which appeared in the August 31, 1946 issue of The New Yorker. The story dealt with the atomic bomb dropped on that Japanese city on August 6, 1945, and its effects on the six Japanese citizens. The article took up the entire issue of the magazine — something The New Yorker had never done before, nor has it since.

The issue of August 31, 1946, arrived in subscribers' mailboxes bearing a light-hearted cover of a summer picnic in a park. There was no hint what lay inside. Hersey's article began where the magazine's regular "Talk of the Town" column ran, immediately following the theater listings.

At the bottom of the page, the editors had appended a short note: "TO OUR READERS. The New Yorker this week devotes its entire editorial space to an article on the almost complete obliteration of a city by one atomic bomb, and what happened to the people of that city. It does so in the conviction that few of us have yet comprehended the all but incredible destructive power of this weapon, and that everyone might well take time to consider the terrible implications of its use. The Editors."

A searing portrait of the bomb and its effects, the article was a publishing sensation. In clear, stripped-down prose, Hersey limned the horrifying aftermath of the atomic device: soldiers' melted eyeballs, citizens instantly vaporized, leaving only their shadows etched onto walls. The issue sold out on newsstands within hours. Requests for reprints poured in to the magazine's offices. The ABC Radio Network preempted regular programming to broadcast the full text in four half-hour programs. Radio stations abroad followed suit. The Book of the Month Club rushed a copy of the article into book format, which it sent to members as a free selection.

Later published by Alfred A. Knopf as a book, Hersey's work is often cited as one of the earliest examples of New Journalism in its melding of elements of non-fiction reportage with the pace and devices of the novel. Hersey's spartan prose was praised by critics as a model of understated narrative. "If ever there was a subject calculated to make a writer overwrought and a piece overwritten, it was the bombing of Hiroshima", wrote Hendrik Hertzberg, "yet Hersey's reporting was so meticulous, his sentences and paragraphs were so clear, calm and restrained, that the horror of the story he had to tell came through all the more chillingly."

The author said he adopted the lean style to suit the story he strove to tell. "The flat style was deliberate", Hersey said 40 years later, "and I still think I was right to adopt it. A high literary manner, or a show of passion, would have brought me into the story as a mediator. I wanted to avoid such mediation, so the reader's experience would be as direct as possible."

Time magazine later called Hersey's account of the bombing "the most celebrated piece of journalism to come out of World War II." Founder of The New Yorker Harold Ross told his friend, author Irwin Shaw: "I don't think I've ever got as much satisfaction out of anything else in my life." But The New Yorker's publication of Hersey's article caused a rift in Hersey's relationship with Henry Luce, the co-founder of Time-Life and Hersey's first mentor, who felt Hersey should have reported the event for one of Luce's magazines instead.

Soon afterward the war correspondent turned mostly to fiction. After publication of Hiroshima, Hersey noted that "the important 'flashes' and 'bulletins' are already forgotten by the time yesterday morning's paper is used to line the trash can. The things we remember are emotions and impressions and illusions and images and characters: the elements of fiction." Shortly before writing Hiroshima, Hersey published his novel Of Men and War, an account of war stories seen through the eyes of soldiers rather than a war correspondent. One of the stories in Hersey's novel was inspired by President John F. Kennedy and the PT-109.

In 1950 Hersey's novel The Wall was published, an account presented as a rediscovered journal recording the genesis and destruction of the Warsaw Ghetto, the largest of the Jewish ghettos established by Nazi Germany during the Holocaust. The book won the National Jewish Book Award in the second year of that award's existence; it also received the Sidney Hillman Foundation Journalism Award.

His article about the dullness of grammar school readers in a 1954 issue of Life magazine, "Why Do Students Bog Down on First R? A Local Committee Sheds Light on a National Problem: Reading" was the inspiration for The Cat in the Hat. Further criticisms of the school system came with "The Child Buyer", a speculative-fiction novel. Hersey also wrote The Algiers Motel Incident, about a racially-motivated shooting by police during the 12th Street Riot in Detroit, Michigan, in 1968. Hersey's first novel A Bell for Adano, which won the Pulitzer Prize for the Novel in 1945, was adapted into the 1945 film A Bell for Adano directed by Henry King starring John Hodiak and Gene Tierney. His 1956 short novel, A Single Pebble is the tale of a young American engineer going up the Yangtze on a river junk in the 1920s and discovering that his romantic concepts of China bring disaster.

From 1965—70, Hersey was Master of Pierson College, one of twelve residential colleges at Yale University, where his outspoken activism and early opposition to the Vietnam War made him controversial with alumni, but admired by students. Following the trial of the Black Panthers in New Haven, Hersey wrote Letter to the Alumni (1970), in which the former Yale College master sympathetically addressed the civil rights and anti-war movements — and attempted to explain them to sometimes-aggravated alumni.

The courtly Hersey also pursued an unusual sideline: he operated the college's small letterpress printing operation, which he sometimes used to turn out broadsides — in 1969 printing an elaborate broadside of an Edmund Burke quote for Yale history professor and fellow residential college master Elting E. Morison.

For 18 years Hersey also taught two writing courses, in fiction and non-fiction, to undergraduates. As Master of Pierson College, he subsequently hosted his old boss Henry Luce — with whom Hersey had reconnected after their falling-out years prior — when Luce spoke to the college's undergraduates. Following Luce's somnolent speech, the former publisher privately revealed to Hersey for the first time that he and his wife Clare Boothe Luce had experimented with LSD under supervision of a physician.

In 1969 Hersey made another sacrifice for his alma mater: he donated the services of his bulldog 'Oliver' as mascot for the Yale football team. Making his debut in the fall of 1969, Handsome Dan XI (the Yale bulldog's traditional moniker) had Hersey concerned about the dog's interest level. A big football fan himself, Hersey had wondered aloud "whether Oliver would stay awake for two hours." With a new mascot at the helm, the sometimes hapless Yale team finished the season with a 7—2 record.

In 1985 John Hersey returned to Hiroshima, where he reported and wrote Hiroshima: The Aftermath, a follow-up to his original story. The New Yorker published Hersey's update in its July 15, 1985, issue, and the article was subsequently appended to a newly-revised edition of the book. "What has kept the world safe from the bomb since 1945 has not been deterrence, in the sense of fear of specific weapons, so much as it's been memory", wrote Hersey. "The memory of what happened at Hiroshima."

A longtime resident of Vineyard Haven, Martha's Vineyard, Massachusetts — chronicled in his 1987 work Blues — John Hersey died at his winter home in Key West, Florida, on March 24, 1993 at the compound he and his wife shared with his friend, writer Ralph Ellison. Hersey's death was front-page news in the following day's New York Times. The writer was buried near his home on Martha's Vineyard. He was survived by his second wife, Barbara (the former wife of Hersey's colleague at The New Yorker, artist Charles Addams, and the model for Morticia Addams), Hersey's five children, one of whom is composer, musician Baird Hersey, and six grandchildren. Barbara Hersey died on Martha's Vineyard 14 years later on August 16, 2007.

On October 5, 2007, the United States Postal Service announced that it would honor five journalists of the 20th century with first-class rate postage stamps, to be issued on Tuesday, April 22, 2008: Martha Gellhorn, John Hersey, George Polk, Rubén Salazar, and Eric Sevareid. Postmaster General Jack Potter announced the stamp series at the Associated Press managing editors meeting in Washington, D.C.In 1968, John Hersey High School in Arlington Heights, Illinois was named in his honor.

Shortly before Hersey's death, then Acting President of Yale Howard Lamar decided the university should honor its longserving alumnus. The result was the annual John Hersey Lecture, the first of which was delivered March 22, 1993, by historian and Yale graduate David McCullough, who noted Hersey's contributions to Yale but reserved his strongest praise for the former magazine writer's prose. Hersey had "portrayed our time", McCullough observed, "with a breadth and artistry matched by very few. He has given us the century in a great shelf of brilliant work, and we are all his beneficiaries."

The John Hersey Prize at Yale was endowed in 1985 by students of the author and former Pierson College master. The prize is awarded to "a senior or junior for a body of journalistic work reflecting the spirit and ideals of John Hersey: engagement with moral and social issues, responsible reportage and consciousness of craftsmanship." Winners of the John Hersey Prize include David M. Halbfinger (Yale Class of 1990) and Motoko Rich (Class of 1991), who both went on to reporting careers at The New York Times, and journalist Jacob Weisberg (Class of 1985), current editor-in-chief of The Slate Group. Among Hersey's earlier students at Yale was Michiko Kakutani, currently the chief book critic of The New York Times, as well as film critic Gene Siskel.

During his lifetime, Hersey served in many posts connected with writing, journalism and education. He was the first non-academic named master of a Yale residential college. He was past president of the Authors League of America, and he was elected chancellor by the membership of the American Academy of Arts and Letters. Hersey was an honorary fellow of Clare College, Cambridge University. He was awarded honorary degrees by Yale University, the New School for Social Research, Syracuse University, Washington and Jefferson College, Wesleyan University, The College of William and Mary and others.Today there is a high school named after John Hersey, located in Arlington Heights IL.