John Leahy was born 7 February 1928, the son of the late William Henry Gladstone and late Ethel Leahy; he was educated at Tonbridge School, Clare College, Cambridge and Yale University. In 1954 he married Elizabeth Anne, daughter of J. H. Pitchford, CBE.

After serving in the RAF, 1950-52, Leahy's diplomatic career included: Foreign Office, 1952-54 (Assistant Private Secretary to Minister of State, 1953-54); 3rd, later 2nd Secretary, Singapore, 1955-57; FO, 1957-58; 2nd, later 1st Secretary, Paris [France] 1958-62; FO, 1962-65; Head of Chancery, Tehran [Iran] 1965-68; Counsellor, Foreign and Commonwealth Office, 1969; Head of Personnel Services Department, 1969-70; Head of News Department, FCO, 1971-73; Counsellor and Head of Chancery, Paris, 1973-75; seconded as Under Secretary, Northern Ireland Office, 1975-76; Assistant Under-Secretary of State, FCO, 1977-79; Ambassador to South Africa, 1979-82; Deputy Under-Secretary of State, FCO, 1982-84; High Commissioner, Australia, 1984-88.

He was awarded the KCMG in 1981 (CMG 1973). Sir John Leahy died on 17 November 2015.