Derek Day was born 29 November 1927, the son of the late Mr and Mrs Alan W. Day; he was educated at Hurstpierpoint College and St Catharine’s College, Cambridge, and in 1955 married Sheila Nott.

After serving in the Royal Artillery, 1946-48, Day entered the Foreign Service in September 1951. During his diplomatic career he served as: Third Secretary, British Embassy, Tel Aviv [Israel], 1953-56; Private Secretary to HM Ambassador, Rome [Italy], 1956-59; Second, then First Secretary, Foreign Office, 1959-62; First Secretary, British Embassy, Washington [United States], 1962-66; First Secretary, FO, 1966-67; Assistant Private Secretary to Secretary of State for Foreign Affairs, 1967-68; Head of Personnel Operations Department, Foreign and Commonwealth Office, 1969-72; Counsellor, British High Commission, Nicosia [Cyprus], 1972-75; Ambassador to Ethiopia, 1975-78; Assistant Under-Secretary of State, 1979, Deputy Under-Secretary of State, 1980, and Chief Clerk, 1982-84, FCO; High Commissioner to Canada, 1984-87.

Day was awarded the KCMG in 1984 (CMG 1973). He died on 7 March 2015.