Adrian Stephen, the youngest of four children of Leslie Stephen and Julia Prinsep Duckworth, was the younger brother of Virginia Woolf and Vanessa Bell. By his mother's first marriage, he was also a half-brother of George and Gerald Duckworth.
After the death of his father in 1904, the four Stephen siblings moving to a house in Bloomsbury, London, which became the nucleus of the Bloomsbury Group. Adrian was educated at Westminster School and then Trinity College, Cambridge, with a degree in law and history. In 1914, he married Karin Costelloe, also a Cambridge graduate. As conscientious objectors during World War I, the couple worked on a dairy farm in Essex during the war years. Towards the end of the war, Stephen and his wife both became interested in the work of Sigmund Freud and psychoanalysis. They trained as physicians and went into analysis with James Glover. In 1936, Stephen wrote a book about the Dreadnought hoax, in which he had taken part 25 years earlier, published by The Hogarth Press founded by Virginia and Leonard Woolf.
In World War II, he abandoned his pacifist stance and volunteered as an army psychiatrist. After the war, he became Scientific Secretary of the British Psychoanalytic Society and took over the job of editor of the International Journal of Psychoanalysis from James Strachey in 1946.