Peter Marshall was born 30 July 1924, 3rd son of R. H. Marshall. He was educated at Tonbridge School and Corpus Christi College, University of Cambridge (Hon. Fellow 1989). In 1957 he married (1st), Patricia Rendell Stoddart (d 1981), having one son and one daughter; 2nd, 1989, Judith (d 2013), widow of E. W. F. Tomlin.

After serving in the Royal Air Force Volunteer Reserve, 1943-46, Marshall joined the Foreign (later Diplomatic) Service in 1949, and in his diplomatic career served in the following posts: Foreign Office, 1949-52; 2nd Secretary and Private Secretary to Ambassador, Washington [United States], 1952-56; Foreign Office, 1956-60; on staff of Civil Service Selection Board, 1960; 1st Secretary and Head of Chancery, Baghdad [Iraq], 1961, and Bangkok [Thailand], 1962-64; Assistant Director of Treasury Centre for Administrative Studies, 1965-66; Counsellor, UK Mission, Geneva [Switzerland], 1966-69, Counsellor and Head of Chancery, Paris [France], 1969-71; Head of Financial Policy and Aid Department, Foreign and Commonwealth Office, 1971-73; Assistant Under-Secretary of State, FCO, 1973-75; UK Representative on Economic and Social Council of United Nations, 1975-79; Ambassador and UK Permanent Representative to Office of United Nations and Other International Organisations at Geneva, 1979-83. He later served as Commonwealth Deputy Secretary General (Economic), 1983-88, and Chairman of the Joint Commonwealth Societies Council, 1993-2003.