Gregg was born at North Cerney, Gloucestershire, England, United Kingdom on 4 July 1873. His elder sister, Hilda Gregg was a popular novelist. He was educated at Bedford School, and at Christ's College, Cambridge, where he was a classical scholar and won the Hulsean Prize Essay competition for 1896 with The Decian Persecution. He came from an Anglo-Irish family, which boasted a large number of Church of Ireland clergy within its ranks. His grandfather, another John Gregg, had sat in the House of Lords as Bishop of Cork, Cloyne and Ross, and his uncle, Robert Samuel Gregg served briefly as Archbishop of Armagh in the 1890s after a long episcopate as Bishop of Cork, Cloyne and Ross. J A F Gregg went on to be a notable church historian. He served as assistant curate of Ballymena under Charles d'Arcy 1896–1899, then as Curate at Cork Cathedral, and as Rector of Blackrock, Co. Cork, before being appointed in 1911 Professor of Divinity in Trinity College, Dublin. In 1915 he became Bishop of Ossory, Ferns and Leighlin, in 1920 Archbishop of Dublin. He was elected to Armagh in 1938, but refused the position largely on account of his wife's health, and Godfrey Day, Bishop of Ossory, was elected in his place. Following Day's death in 1939, Gregg was again elected Archbishop of Armagh which post he held until his retirement in 1959. He was married twice. First in 1902 to Anna Jennings by whom he had two sons and two daughters, and secondly, in 1947, to Lesley McEndoo, younger daughter of the then Dean of Armagh. His daughter, Barbara, was a novelist. He was a supporter of the old Unionist order but encouraged his flock to make their peace with the post-1922 political realities in Ireland. According to R. B. McDowell –
"...the Church of Ireland was led by John Allen Fitzgerald Gregg, archbishop successively of Dublin and Armagh, who might fairly be described as an instinctive conservative with, however, an awareness of contemporary trends... Gregg's bearing suggested a prince of the church or at least a prelate of the establishment... he was a scholar and a man of affairs, his administrative flair being reinforced by dignity, decisiveness, and a sardonic wit... His theological sympathies were high church, though he had been brought up an evangelical and had an Anglo-Irish distaste for ceremonial exuberance.
