Younger son of Philip St George Mansergh and Mrs E. M. Mansergh, born on 27 June 1910 in Tipperary, Ireland. Mansergh was educated at Abbey School, Tipperary and St Columba's College, Dublin (1923-9). He then entered Pembroke College, Oxford, in 1929, where he read modern history. Despite not managing to obtain a first, he began postgraduate research under W.G.S Adams (Gladstone professor of political theory and institutions). He achieved his DPhil in 1936, and was subsequently appointed as a tutor (but not Fellow) in politics at Pembroke. This post enabled him to produce a major work, Ireland in the Age of Reform and Revolution (1940). At this time he was also secretary to the Oxford University Politics Research Committee. In 1939 Mansergh married Diana Mary Keeton (undergraduate at Lady Margaret Hall, daughter of the headmaster of Reading School) on 12 December 1939, and their marriage produced 5 children (3 sons and 2 daughters).
During the Second World War , Mansergh became the Irish expert and director of the empire division of the Ministry of Information, which led to his appointment as OBE in 1945, and then as an assistant secretary at the Dominions Office (1946-7). After this foray into the civil service, Mansergh returned to academic life in 1947 as a research professor at the Royal Institute of International Affairs. In 1953 he moved to Cambridge as the first Smuts Professor of the history of the British Commonwealth. From this position, which he held from 1953-1970, Mansergh was concerned to raise the profile of the study of both Irish and Commonwealth history. During this time, Mansergh also became an honorary fellow of Pembroke College, Oxford (1954), and Trinity College, Dublin (1971), obtained an Oxford DLitt in 1960, as well as a fellowship of the British Academy in 1973. He was Master of St John's College from 1969-1979, after which he returned to being a Fellow until his death in 1991. Perhaps his greatest work was the publication, as editor-in-chief, of the 12-volume, highly-acclaimed series Transfer of Power in India, 1942-7 (TOPI), which appeared from 1970 at the rate of one a year.
Mansergh died at Brookfields Hospital, Cambridge, on 16 January 1991, from pneumonia which set in at the end of a prolonged period of ill health which was unfortunately begun by a fall on an escalator of the London underground.