After graduation, Mansergh was a tutor in the school of
Modern Greats at the
University of Oxford and secretary to the
Oxford Union Politics Research Committee. 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. In an interview a half century later, Mansergh noted: The Commonwealth for my generation had something in common with the Common Market nowadays. I was interested in the Commonwealth to see if it would provide a way forward in Ireland itself. An inherent weakness in the Anglo-Irish Treaty was that the Dominion settlement was not consistent with Partition [from Northern Ireland]. I felt that Dominion status wouldn't work, which was obvious enough by 1934, but I wasn't sure whether any alternative to Dominion status would work in Ireland's case. Mansergh followed this up in 1940 with
Ireland in the Age of Reform and Revolution, which critically analysed the
Marxist dialectic as it had been applied to Ireland, noting later that this led to his frequent misidentification as a
Marxist historian. After the war, Mansergh was appointed Abe Bailey Professor of British Commonwealth relations at
Chatham House (1945-1953). He also began visiting India as an observer at the
Asian Relations Conference. Upon his return, Mansergh gave a lecture on "The Implications of Éire's Relations with the British Commonwealth of Nations", which helped influence Commonwealth relations during the late 1940s. W. K. Hancock states: "In the departments of history in nearly every British university Ireland had remained for too long a forgotten country, except as an irritating intruder into British party politics. Cambridge had been in some degree an exception to that bad rule.. 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." In 1967 he was appointed editor-in-chief by the prime minister,
Harold Wilson, of a multi-volume collection of documents from the
India Office on the transfer of power to India in the 1940s. 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). In 1971 he was made an honorary fellow of
Trinity College Dublin. 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. Historian Margaret O'Callaghan said "Nicholas Mansergh...was one of the finest historians of high political relations between successive British governments and those of the two parts of the island of Ireland." He was a member of the British Academy, where the obituary by David Harkness praised his distinguished work. ==Personal life==