He was a fellow of Balliol College, Oxford (1925–44), professor of history at the University of Edinburgh (1944–45) and warden of All Souls College, Oxford (1945–51).
The son of the artist Heywood Sumner and his wife Agnes (a sister of Lord Charnwood), he attended Winchester College and Balliol College, Oxford.
By that time he had already been appointed a fellow of All Souls College, Oxford (in 1919) and, after his spell in the ILO ended in 1922, he was appointed a tutor at Balliol College; he was elected to a fellowship there in 1925.
He remained there until 1944, when he was appointed professor of history at the University of Edinburgh, but the following year was appointed warden of All Souls College, in which office he remained until his death in 1951.
He was also a pioneering expert in Russian history and combined his two interests by advancing British scholarship on the study of diplomacy in Slavonic Europe.