Derek Thomas was born on 31 October 1929, the son of K. P. D. Thomas and Mali McL. Thomas. He was educated at Radley College, Abingdon, and Trinity Hall, Cambridge. In 1956 he married Lineke van der Mast; they had two children.
Thomas entered the Foreign Service initially in 1953, then served as a Midshipman (1953) and then Sub-Lieutenant (1955) in the Naval Reserve. Returning to the Foreign Office in 1955, Thomas's diplomatic career included: 3rd, later 2nd, Secretary, Moscow [Soviet Union], (1956-59); 2nd Secretary, Manila [Philippines], (1959-61); UK Delegation to Brussels Conference [Belgium], (1961-62); 1st Secretary, FO, 1962; Sofia [Bulgaria], (1964-67); Ottawa [Canada], (1967-69); seconded to Treasury, (1969-70); Financial Counsellor, Paris [France], (1971-75); Head of North American Department, Foreign and Commonwealth Office, (1975-76); Assistant Under Secretary of State, FCO, (1976-79); Minister Commercial and later Minister, Washington [United States], (1979-84); Deputy Under Secretary of State for Europe and Political Director, FCO, (1984-87); Ambassador to Italy, (1987-89).