William Strang was born in Rainham, Essex, 2 January 1893, the son of James Strang and Margaret Strang (née Steven). He was educated at Palmer's School in Grays, Essex; University College London; and the Sorbonne (University of Paris). He married Elsie Wynne Jones, 1 March 1920, with whom he had a daughter and a son.

During the First World War, he served with the 4th battalion of the Worcestershire Regiment and on the headquarters staff of the 29th Division. In 1919, he joined the Diplomatic Service and was posted to Belgrade, where he was Third Secretary, 1919-20, Second Secretary, 1920, and acting Chargé d'Affaires, 1921-2. He returned to the Foreign Office in London in 1923, where he worked in the Northern Department and was an expert on the Soviet Union. He attended the Anglo-Soviet Conference in 1924, was promoted to First Secretary in 1925, and posted to Moscow, as Acting Counsellor, 1930-2, and Counsellor, 1932-3. On returning to London, he became head of the League of Nations Section, 1933-7, and then of the Central Department, dealing with Germany, 1937-9.

He was Assistant Under-Secretary of State in the Foreign Office, overseeing relations with occupied European countries, 1939-43, and then British representative on the European Advisory Commission with the personal rank of Ambassador, 1943-5. After the war, he became Political Adviser to the Commander-in-Chief of the British army of occupation in Germany, 1945-7, and Joint Permanent Under-Secretary in the Foreign Office, in charge of the German Section, 1947-9. He was appointed Permanent Under-Secretary in 1949 and retired from the Foreign Office in 1953.

He was appointed MBE, 1918; CMG, 1932; CB, 1939; KCMG, 1943; KCB, 1948; GCMG, 1950; and GCB, 1953. He was created a hereditary peer, as Baron Strang, in 1954.

As a member of the House of Lords, he was Deputy Speaker and Deputy Chairman of Committees, 1962. In retirement, he was also Chairman of the National Parks Commission, 1954-66; a member of the Nature Conservancy Council, 1954-66; Chairman of the Food Hygiene Advisory Council, 1955-71; and Chairman of the Royal Institute of International Affairs, 1958-65. In 1946, he became a Fellow of University College London; he was Chairman of the College Committee, 1963-71; and, in 1954, he was awarded an honorary doctorate by London University.

His publications include: The Foreign Office (1955); a memoir, Home and Abroad (1956); a history, Britain in World Affairs (1961); and Diplomatic Career (1962).

He died in Newcastle, 27 May 1978.