Trevor-Roper graduated from Christ Church College, Oxford, in 1936, and in 1939, as a research fellow at Merton College, he qualified for the M.A. degree. His first book was Archbishop Laud, 1573–1645 (1940), a biography of the archbishop of Canterbury and adviser to King Charles I. During World War II, Trevor-Roper was an intelligence officer and helped investigate Hitler's death. In 1947 his book The Last Days of Hitler was published, and it quickly became a best-seller. From 1946 to 1957 he taught history at Christ Church College. During this period he wrote several articles about Hitler (Hitler, Adolf), stirring controversy by contending that Hitler was not only a systematic thinker but a genius as well. In 1957 he was appointed regius professor of modern history and fellow of Oriel College, Oxford. He remained at this post until 1980, when he was appointed Master of Peterhouse College, Cambridge, where he stayed until 1987. He was created a life peer in 1979.

His interest in modern history is evident in his works about World War II. He wrote The Philby Affair: Espionage, Treason, and Secret Service (1968) and edited Hitler's Table Talk, 1941–1944 (1953), Hitler's War Directives, 1939–1945 (1964), and The Goebbels Diaries (1978). He gained international attention in 1983 when he certified as genuine some 60 volumes of diaries purported to be Hitler's; they later proved to be forgeries.

Trevor-Roper wrote a number of noncontroversial books, including The Gentry, 1540–1640 (1953), Historical Essays (1957), Religion, the Reformation and Social Change, and Other Essays (1967), Princes and Artists: Patronage and Ideology at Four Habsburg Courts, 1517–1633 (1976), and Catholics, Anglicans and Puritans (1987). He also wrote a biography revealing the amazing deceptions of Sir Edmund Backhouse, an internationally known Sinologist.