John Margetson was born 9 October 1927, younger son of the Very Rev. W. J. Margetson and Marion Jenoure. He was educated at Blundell's, then at St John's College, University of Cambridge. In 1963 he married Miranda, daughter of Sir William Menzies Coldstream, CBE and Mrs Nancy Spender, having one son and one daughter.
After his National Service as a Lieutenant in the Life Guards, 1947-49, Margetson initially joined the Colonial Service, as District Officer for Tanganyika [later Tanzania], 1951-60 (Private Secretary to the Governor, Sir Edward Twining, 1956-57). He then entered the Foreign Service in 1960, and his diplomatic career included the following posts: The Hague, 1962-64; speech writer to Foreign Secretary, George Brown, 1966-68; Head of Chancery, Saigon, 1968-70; Counsellor 1971, seconded to Cabinet Secretariat, 1971-74; Head of Chancery, UK Delegation to NATO, 1974-78; Ambassador to Vietnam, 1978-80; seconded to Ministry of Defence as Senior Civilian Instructor, Royal College of Defence Studies, 1981-82; Ambassador and Deputy Permanent Representative to United Nations, New York [United States], and President, UN Trusteeship Council, 1983-84; Ambassador to the Netherlands, 1984-87; Special Representative of the Foreign Secretary, 1994-98.