Mansergh, (Philip) Nicholas Seton (1910–91), historian, was born 27 June 1910 at his family home, Greenane House, Tipperary, the younger son of Philip St George Mansergh, railway engineer (in Australia and east Africa) and farmer, and his wife, Ethel Marguerite Otway Louise Mansergh; his parents were cousins. He was educated at the Abbey School, Tipperary, St Columba's College, Dublin (1923–9), and at Pembroke College, Oxford. Having read modern history, he graduated in 1932 with a disappointing second-class degree, but went on to do postgraduate research. Influenced by his research supervisor, Professor W. G. S. Adams, who had been adviser to Lloyd George on Irish matters, his work was more political science than history. He was awarded a B.Litt. in 1933, and a D.Phil. in 1936, the theses for which provided the basis for two pioneering studies of contemporary Ireland: The Irish Free State: its government and politics (1934), and The government of Northern Ireland: a study in devolution (1936). Appointed tutor in politics at Pembroke College in 1937, he began work on his next book, Ireland in the age of reform and revolution, published in 1940 (and in a revised edition as The Irish question, 1840–1921 in 1975).
In 1941 Mansergh joined the empire division of the Ministry of Information, where he served as their expert on Irish affairs. He remained a civil servant (transferring to the dominions office in 1946) until 1947, when he returned to academic life, becoming research professor at the Royal Institute of International Affairs, and, from 1953 to 1970, the first Smuts professor of the history of the British commonwealth at Cambridge. He was elected a fellow of St John's College (where he was affectionately known as ‘Old Nick’) in 1955, and served as master, 1969–79.
Mansergh was an extremely productive scholar. Among his publications were two volumes in the Survey of British Commonwealth Affairs series, Problems of external policy 1931–1939 (1952), and Problems of wartime co-operation and post-war change, 1939–1952 (1958), along with two supporting volumes, Documents and speeches on commonwealth affairs, 1931–1952 (1953); South Africa, 1906–1961: the price of magnanimity (1962); Documents and speeches on British commonwealth affairs, 1952–62 (1963); The commonwealth experience (1969; new ed., 2 vols, 1982); some three dozen articles and essays; and, as editor-in-chief, twelve volumes of official documents, The transfer of power in India, 1942–7 (1970–82). The unresolved question: the Anglo-Irish settlement and its undoing, 1912–72 (1991) was published after his death, as were two volumes of essays, edited by his widow, Diana: Nationalism and independence: selected Irish papers (1997), and Independence years: the selected Indian and commonwealth papers of Nicholas Mansergh (1999). As a teacher he lectured on commonwealth history, introduced (for the first time) a twentieth-century Irish history course into the undergraduate syllabus, established Cambridge's first modern-style research seminar, and supervised doctoral research students.
Mansergh has a secure and enduring reputation as a historian in three distinct fields: the commonwealth as a whole, India, and Ireland. While his surveys of commonwealth affairs were masterly exercises in contemporary history, The commonwealth experience traced the development of the British imperial system from 1839 until the emergence after the second world war of a multi-racial organisation, sharing a common ‘British’ experience, and based on an assumption of equality and partnership, though not without its problems and crises. Although India played a crucially important part in the evolution of the ‘new’ commonwealth after 1947, Mansergh's reputation as a student of India is primarily based on the ‘transfer of power’ documents, which one (Indian) reviewer declared would ‘remain for all time to come an indispensable reference work’ (Professor Bimal Prasad, International Studies, Delhi). Two central concerns of Mansergh's were combined in his 1976 Smuts memorial lecture (published in 1979 as The prelude to partition: concepts and aims in Ireland and India), in which he illuminatingly explored the comparative resonances of partition, and the sometimes conflicting pressures for nationalists of the pursuit of both independence and unity.
Ireland, above all, was central to Mansergh's scholarship, and, appropriately, formed the subject of his first and last monographs. Ireland in the age of reform and revolution, and its revised version, The Irish question, treated Anglo–Irish relations in an explicitly comparative framework, stylishly exploring British, American, and continental European dimensions. It is a testament to the work's lucid and dispassionate quality, as well as its pioneering brilliance, that over fifty years after it was first published it was still regarded as an essential text for the understanding of nineteenth- and early twentieth-century Ireland. One of Mansergh's great strengths as a historian was his passionate conviction that history was not merely about the ‘past’. The violent and painful experience of Ireland, especially in the twentieth century, placed a special responsibility on the historian to assist the understanding of the circumstances that had led to this situation. In the 1975 preface to The Irish question, at a time when serious violence had afflicted Northern Ireland for some years, Mansergh modestly expressed his hope that the approach that he had adopted in the book would prove ‘fruitful’ to ‘an assessment of the nature of the Irish question and to an understanding of the difficulties of resolving it peacefully’.
The relating of ‘history’ to the violent politics of modern and contemporary Ireland is made explicit in The unresolved question, which extended Mansergh's commentary on Anglo–Irish relations to the twentieth century. Reflecting on his own experience as a schoolboy in Tipperary, close to Soloheadbeg, where the first shots of the 1919–21 Irish war of independence were fired, he wrote: ‘the author may perhaps be relied upon to think of the events of those days as near realities, not as distant phenomena or as issues in high politics’. In a typically suggestive and elegant passage he reminded his readers that ‘history was forged in sudden death on a Tipperary by-road as surely as ever it was in meetings in Downing Street or for that matter at the Mansion House in Dublin, where the dáil met coincidentally but fortuitously for the first time that same day, 21 January 1919’. Powerfully informed by Mansergh's own humane and liberal sympathies, the book is an indispensable guide to relations between Britain and Ireland from the home rule crisis before the first world war to the formal secession of Ireland from the British commonwealth in 1949.
At a distance, Nicholas Mansergh gave an impression of professorial austerity, which wholly belied his thoughtful, kindly, and good-humoured character. He was generous with his time and his unrivalled scholarship, even to the most junior of colleagues. In 1939 he married Diana Mary Keeton, daughter of the late G. H. Keeton, who had been headmaster of Reading School. They met at Oxford, while Diana was an undergraduate at Lady Margaret Hall and Nicholas was in search of a mixed-doubles partner for tennis, a game at which he excelled. They had two daughters and three sons, one of whom, Martin Mansergh, became a senior public servant in Ireland, a special adviser to the taoiseach, a member of Seanad Éireann (2002–2007) and was elected TD for Tipperary South in 2007. Mansergh died on 16 January 1991. There is a portrait (1973) of him by William Narraby in St John's College, Cambridge.