After graduating, he remained at the University of Oxford. From 1946 to 1947, he was a Bryce Research Student. He then worked as a lecturer at University College, Oxford between 1947 and 1950. In 1950, he was elected a Fellow of Wadham College, Oxford. As such, he was a college tutor in history, and moved specialising in medieval history to Tudor history. For two years, from 1960 to 1961, he was also a member of the Institute for Advanced Study in Princeton, New Jersey.