Denis Arthur Greenhill, Baron Greenhill of Harrow GCMG OBE (1913–2000) was the British Permanent Under-Secretary of State for Foreign Affairs and Head of the Diplomatic Service from 1969 to 1973; a respected expert on the US, Europe and the Soviet Union, he was actively involved in setting postwar Britain's role in the world in a new direction, away from its imperial past and a compliant involvement with the United States towards a more active engagement in Europe. He served under three prime ministers, Harold Wilson, Sir Alec Douglas Home and Edward Heath.
Greenhill was educated at Bishop's Stortford College and Christ Church, Oxford. From 1935-39, he worked for the London and North Eastern Railway. During World War II, he served in the Middle East, Far East, India and North Africa. He was promoted colonel in the Royal Engineers and was mentioned in despatches twice,.
His wife, Angela (née McCulloch), whom he married in Cairo in 1941, died in 2013.
He joined the Foreign Office in 1946 with the support of the Labour Foreign Secretary Ernest Bevin, who thought him an expert on oil transportation. He served as counsellor (and later minister) at the British Embassy in Washington, at the time of the Cuban missile crisis (1962) and the assassination of U.S. President John F. Kennedy (1963). As Britain's special envoy to Rhodesia (1972, 1976) he sought unsuccessfully to end Ian Smith's unilateral declaration of independence. He was PUS at the time of Britain's entry into the European Community (1973). His appointment as PUS had been regarded with scepticism by older hands in the Whitehall establishment.