He was born in Glanton, Northumberland, and educated at Charterhouse and at Christ Church, Oxford. In 1957 he was appointed Regius Professor of Modern History at the University of Oxford, a post he held until 1980; subsequently he became Master of Peterhouse, Cambridge. Having achieved his first major success with The Last Days of Hitler (1947), he consolidated his reputation as an authority on the Third Reich with books such as Hitler's Table Talk (1953) and The Goebbels Diaries (1978), although his area of specialty was Early Modern Britain, especially the period around the English Civil War.
On October 4, 1954, he married Lady Alexandra Henrietta Louisa Howard-Johnston (March 9, 1907 - August 15, 1997), eldest daughter of Field Marshal the 1st Earl Haig by his wife, the former Hon. Dorothy Maud Vivian. Lady Alexandra was a goddaughter of Queen Alexandra, and had previously been married to Rear-Admiral Clarence Dinsmore Howard-Johnston, by whom she had had three children.
The nadir of his career came in 1983, when, along with others, he authenticated the so-called Hitler Diaries, which later forensic examination proved to be a fake. This raised questions in the public mind not only about his perspicacity as a historian but also about his personal integrity, because The Sunday Times, a newspaper to which he regularly contributed book reviews and in whose parent company he held a financial interest, had already shelled-out a considerable sum for the right to serialise the diaries. He denied any dishonest motivation, insisting that he, like others, had made a genuine mistake. Despite the shadow that this incident cast over his later career, he continued writing (producing Catholics, Anglicans, and Puritans in 1987, for example), and his work continued to be well received.
He died of cancer in a hospice in Oxford, aged 89.