was born at Rosebank Cottage in Cardross, Dunbartonshire, the only child of a Protestant mother, Jessie Cronin (née Montgomerie), and a Catholic father of Irish extraction, Patrick Cronin, and would later write of young men from similarly mixed backgrounds. His paternal grandparents were the proprietors of a public house in Alexandria, West Dunbartonshire. His maternal grandfather, Archibald Montgomerie, was a hatter who owned a shop in Dumbarton. After their marriage, Cronin's parents moved to Helensburgh, where he attended Grant Street School. When he was seven years old, his father, an insurance agent and commercial traveller, died from tuberculosis. He and his mother moved to her parents' home in Dumbarton, and she soon became the first female public health inspector in Scotland.

Cronin was not only a precocious student at Dumbarton Academy who won many prizes and writing competitions, but also an excellent athlete and football. From an early age, he was an avid golfer, a sport he enjoyed throughout his life, and he loved salmon fishing as well. The family later moved to Yorkhill, Glasgow, where he attended St Aloysius College in the Garnethill area of the city. He played football for the First XI there, an experience he included in one of his last novels, The Minstrel Boy. A family decision that he should study for either the church or medicine was settled by Cronin himself, who chose "the lesser of two evils." He won a Carnegie scholarship to study medicine at the University of Glasgow in 1914. He was absent during the 1916-1917 session for naval service. In 1919 he graduated with highest honours, with the degree of MB, ChB. Later that year he made a trip to India as ship's surgeon on a liner. Cronin went on to earn additional degrees, including a Diploma in Public Health (1923) and his MRCP (1924). In 1925, he was awarded an M.D. from the University of Glasgow for his dissertation, entitled "The History of Aneurysm."

During World War I Cronin served as a Surgeon Sub-Lieutenant in the Royal Navy Volunteer Reserve before graduating from medical school. After the war, he trained at various hospitals including Bellahouston and Lightburn Hospitals in Glasgow and Rotunda Hospital in Dublin, before taking up his first practice in Tredegar, a mining town in South Wales. In 1924, he was appointed Medical Inspector of Mines for Great Britain, and over the next few years, his survey of medical regulations in collieries and his reports on the correlation between coal dust inhalation and pulmonary disease were published. He was involved in the disaster at Ystfad Colliery in Pengelly, where thirty-eight miners were drowned and the remaining twenty-three were rescued after eight days. Cronin drew on this haunting experience and his research of the occupational hazards of the mining industry for his later novels The Citadel, set in Wales, and The Stars Look Down, set in Northumberland. He subsequently moved to London, where he practised in Harley Street before opening his own thriving medical practice in Westbourne Grove, Notting Hill. Cronin was also the medical officer for Whiteleys at this time and was becoming increasingly interested in ophthalmology.

In 1930, after being diagnosed with a chronic duodenal ulcer, Cronin was told he must take six months complete rest in the country on a milk diet. At Dalchenna Farm on Loch Fyne, he was finally able to indulge his lifelong desire to write a novel, having theretofore "written nothing but prescriptions and scientific papers". From Dalchenna Farm he travelled to Dumbarton to research the background of the novel, using the files of Dumbarton Library, which still has the letter from Cronin requesting advice on this. He composed Hatter's Castle in the span of three months, and the manuscript was quickly accepted by Gollancz, the only publishing house to which it had been submitted (apparently chosen when his wife randomly stuck a pin into a list of publishers). This novel, which was an immediate and sensational success, launched his career as a prolific author, and he never returned to practising medicine.

Many of Cronin's books were bestsellers which were translated into numerous languages. His strengths included his compelling narrative skill and his powers of acute observation and graphic description. Although noted for its deep social conscience, his work is filled with colourful characters and witty dialogue. Some of his stories draw on his medical career, dramatically mixing realism, romance, and social criticism. Cronin's works examine moral conflicts between the individual and society as his idealistic heroes pursue justice for the common man. One of his earliest novels, The Stars Look Down, chronicles transgressions in a mining community in Northeast England and an ambitious miner's rise to be a Member of Parliament. Cronin's humanism continues to inspire - the film Billy Elliot was partly drawn from The Stars Look Down, and the opening song of Billy Elliot the Musical is entitled thus as a tribute. Through the years, Cronin's body of work has influenced many people to become writers or to enter the medical profession, including Giorgio Armani, who planned to become a doctor before becoming a designer.

A prodigiously fast writer, Cronin liked to average 5,000 words a day, meticulously planning the details of his plots in advance. He was known to be tough in business dealings, although in private life he was a person whose "pawky humour...peppered his conversations," according to one of his editors, Peter Haining.

Cronin also contributed a large number of stories and essays to various international publications.

The Citadel, a tale of a mining company doctor's struggle to balance scientific integrity with social obligations, incited the establishment of the National Health Service in the United Kingdom by exposing the inequity and incompetence of medical practice at the time. In the novel, Cronin advocated a free public health service in order to defeat the wiles of those doctors who "raised guinea-snatching and the bamboozling of patients to an art form." Dr. Cronin and Aneurin Bevan had both worked at the Tredegar Cottage Hospital in Wales, which served as the basis for the NHS. The author quickly made a number of enemies in the medical profession, and there was a concerted effort by one group of specialists to get The Citadel banned. Cronin's novel, which was the publisher's best-selling book in its history, informed the public of corruption within the medical system, planting a seed that eventually led to necessary reform. Not only were the author's pioneering ideas instrumental in the creation of the NHS, but historian Raphael Samuel has stated that the popularity of his novels played a substantial role in the Labour Party's landslide 1945 victory.

By contrast, according to one of Cronin's biographers, Alan Davies, the book's reception was mixed. A few of the more vociferous medical practitioners of the day took exception to one of its many messages; that a few well-heeled doctors in fashionable practices were ripping off their equally well-off patients. Some, more sensibly, pointed to the lack of balance between criticism and praise for hard working doctors. The majority accepted it for what is was - a perceptive, topical novel. The press, typically, attempted to incite passions within the profession in an attempt to sell copy, while Victor Gollancz, Cronin's publisher, followed suit in an attempt to promote the book; all conveniently (or purposely) overlooking the fact that it was a work of fiction, not a scientific piece of research, and not autobiographical.

A number of Cronin's novels also deal with religion, something he had grown away from during his medical training and career, and with which he reacquainted himself in the thirties. At medical school, as he recounts in his essay "Why I believe in God" he had become an agnostic: "When I thought of God it was with a superior smile, indicative of biological scorn for such an outworn myth". During his practice in Wales, however, the deep religious faith of the people he worked among made him start to wonder whether "the compass of existence held more than my text-books had revealed, more than I had ever dreamed of. In short I lost my superiority, and this, though I was not then aware of it, is the first step towards finding God.

He also came to feel that "If we consider the physical universe,... we cannot escape the notion of a primary Creator.... Accept evolution with its fossils and elementary species, its scientific doctrine of natural causes. And still you are confronted with the same mystery, primary and profound. Ex nihilo nihil, as the Latin tag of our schooldays has it: nothing can come of nothing." This was brought home to him in London, where in his spare time he had organized a working boys' club. One day he invited a distinguished zoologist to deliver a lecture to the members. The speaker, adopting "a frankly atheistic approach," described the sequence of events leading to the emergence, "though he did not say how," of the first primitive life-form from lifeless matter. When he concluded, there was polite applause. Then, "a mild and very average youngster rose nervously to his feet" and with a slight stammer asked how all that matter had got there in the first place. The naïve question took everyone by surprise. The lecturer "looked annoyed, hesitated, slowly turned red. Then, before he could answer, the whole club burst into a howl of laughter. The elaborate structure of logic offered by the test-tube realist had been crumpled by one word of challenge from a simple-minded boy."

It was at university that Cronin met his future wife, Agnes Mary Gibson (May), who was also a medical student. She was the daughter of Robert Gibson, a master baker, and Agnes Thomson Gibson (née Gilchrist) of Hamilton, Lanarkshire. The couple married on 31 August 1921. As a doctor, May worked with her husband briefly in the dispensary while he was employed by the Tredegar Medical Aid Society; she also assisted him with his practice in London. When he became an author, she would proofread his manuscripts. Their first son, Vincent, was born in Tredegar in 1924. Their second son, Patrick, was born in London in 1926. Andrew, their youngest son, was born in London in 1937.

With his stories being adapted to Hollywood films, Cronin and his family moved to the United States in 1939, living in Bel-Air, California, Nantucket, Massachusetts, Greenwich, Connecticut, and Blue Hill, Maine. In 1945, the Cronins sailed back to England aboard the RMS Queen Mary, where they stayed briefly in Hove and then in Raheny, Ireland before returning to the U.S. the following year. They subsequently took up residence at the Carlyle Hotel in New York City and then in Deerfield, Massachusetts before settling in New Canaan, Connecticut in 1947. Ever the nomad, Cronin also frequently travelled to his homes in Bermuda and Cap-d'Ail, France, where he summered.

Ultimately, Cronin returned to Europe, residing in Lucerne and Montreux, Switzerland for the last 25 years of his life and continuing to write into his eighties. He included among his friends Lord Olivier, Sir Charles Chaplin and Audrey Hepburn, to whose first son he was godfather.

Although the latter part of his life was spent entirely abroad, Cronin retained a great affection for the district of his childhood, writing in 1972 to a local teacher: "Although I have travelled the world over I must say in all sincerity that my heart belongs to Dumbarton.... In my study there is a beautiful 17th century coloured print of the Rock.... I even follow with great fervour the fortunes of the Dumbarton football team." Further evidence of Cronin's lifelong support of Dumbarton F. C. comes from a framed typewritten letter hanging in the foyer of the club's stadium. In the letter, written in 1972 and addressed to the club's then secretary, Cronin congratulates Dumbarton on their return to the top division after an absence of 50 years and recalls his childhood supporting the Sons (the club's nickname) and on occasion being "lifted over" the turnstiles (a common practice in times past so that children did not have to pay).

Cronin died on 6 January 1981 in Montreux, and is interred at La Tour-de-Peilz. Many of Cronin's writings, including published and unpublished literary manuscripts, drafts, letters, school exercise books and essays, laboratory books, and his M.D. thesis, are held at the National Library of Scotland and at the Harry Ransom Center at the University of Texas.

• Salwak, Dale. A. J. Cronin. Boston: Twayne's English Authors Series, 1985. ISBN 0-8057-6884-X

• Davies, Alan. A. J. Cronin: The Man Who Created Dr Finlay. Alma Books, October 2010. ISBN 9781846881121