Philip Nicholas Seton Mansergh, OBE (27 June 1910 – 16 January 1991) was a distinguished historian of Ireland and the British Commonwealth.
Nicholas Mansergh was born at Greenane House, Tipperary, Ireland. He was the second son of Philip St George Mansergh (1863–1928), a railway engineer, and Ethel Marguerite Otway Louise Mansergh (1876–1963). 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. After a short period at school in the north, Mansergh attended the Erasmus Smith (Abbey) School in his native Tipperary, which was founded in 1760. He was the youngest boy there when the school suddenly closed in 1922. After the Irish Civil War, Mansergh attended St. Columba's College, Dublin with his elder brother, then he went up to Pembroke College, Oxford to read modern history. There he came under the influence of R. B. McCallum and was later supervised by W.G.S. Adams.
After graduation, Mansergh was a tutor in the school of Modern Greats at 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. During the Second World War, Mansergh worked in the British Ministry of Information, where after working on Anglo-Irish information services and cultural relations he was appointed head of the Empire division in 1944. He was made an Officer of the Order of the British Empire in the 1946 New Year Honours.