Comfort was educated at Highgate School and Trinity College, Cambridge. He studied medicine at the University of Cambridge (pre-clinical study leading to a BA, upgraded in 1944 to an MA) and the London Hospital (now known as the Royal London Hospital), qualifying in 1944 with both the Conjoint diplomas of LRCP London, MRCS England and the Cambridge MB BChir degrees.

Comfort served as a House Physician at the London Hospital and went on to become a lecturer in physiology at the London Hospital Medical College. In 1945 he obtained the Conjoint Board's Diploma in Child Health, and progressed to a PhD in 1950 and a DSc of University College, London in 1963.

A leading pacifist, Comfort considered himself "an aggressive anti-militarist", and he believed that pacifism rested "solely upon the historical theory of anarchism". He was an active member of CND.

Among the works on anarchism by Comfort is Peace and Disobedience (1946), one of many pamphlets he wrote for Peace News and the Peace Pledge Union, and Authority and Delinquency in the Modern State (1950). He exchanged public correspondence with George Orwell defending pacifism in the open letter/poem "Letter to an American Visitor" under the pseudonym "Obadiah Hornbrooke."

He had a successful academic career in both England and the United States of America, in parallel with his social and political activism, and was a prolific writer.

Comfort's 1972 book The Joy of Sex earned him worldwide fame and $3 million. But he was unhappy to become known as "Dr. Sex" and to have his other work given so little relative attention.

Comfort devoted much of the 1950s and 1960s studying the biology of aging (biogerontology) as well as popularizing the subject. He could be called an early biomedical gerontologist (life extensionist) on the basis of his view that science could extend human lifespan. In 1969 he suggested that life expectancy (not simply maximum life span) could be extended to 120 within 20 years. Although Comfort believed that aging could be postponed, he did not believe that it could be eliminated, and he did not write about rejuvenation.

In old age he returned to England, and in his last years was disabled after a stroke. He died aged 80 on 26 March 2000 in South Northamptonshire.