One of his earliest memories was of trains leaving the town carrying soldiers destined for service on the Western Front in the First World War.
His first book, The Irish Free State: Its Government and Politics (1934), fuelled his subsequent interest in the Commonwealth, one that he would pursue for the remainder of his academic career.
An inherent weakness in the Anglo-Irish Treaty was that the Dominion settlement was not consistent with Partition [from Northern Ireland].
[5][6] After the war, Mansergh was appointed Abe Bailey Professor of British Commonwealth relations at Chatham House (1945-1953).
[4] In 1953 Mansergh was appointed to the newly created position of Smuts Professor of the History of the British Commonwealth at Cambridge University.
But Mansergh was the first member of the faculty to make specific provisions for the teaching of Irish history both to undergraduates and graduates.... For as far ahead as anybody can foresee, Mansergh's contribution to Irish historiography will remain an enlightening and civilized influence upon intelligent teachers, students and men of affairs both in Ireland and in Britain.
Two years later, he published one of his most important works, The Commonwealth Experience, and was elected Master of St John's College, Cambridge (1969-1979).
[9] He served as Master until 1979, and continued there afterwards as a fellow, and he was also three times Visiting Professor at the Indian School of International Studies in New Delhi.
Diana Keeton had studied languages at Lady Margaret Hall, Oxford and was a women's squash and lawn tennis blue.
St John's College awards an annual Mansergh Prize in his honour to the best short dissertation or essay (under 10,000 words) on history.