 In 1947 he was appointed as Professor of Medieval History and then, in 1954, he became the Regius Professor of Modern History, a post he held until his retirement in 1963.He served as president of the Royal Historical Society from 1957 to 1961; and was the first President of the Ecclesiastical History Society (1961–63).While pursuing his academic life at Cambridge, Knowles was eventually, at the instigation of Abbot Christopher Butler, exclaustrated from Downside Abbey and finally released from his vows.