Strang was commissioned into the Worcestershire Regiment in 1915 and served in the First World War. He ended the war as a Captain. In 1919, he joined the Diplomatic Service and served at the British embassy in Belgrade from 1919 to 1922, at the Foreign Office from 1922 to 1930 and at the embassy in Moscow from 1930 to 1933. During his time in Moscow he played an important role in the Metro-Vickers engineers trial, in which six British engineers were accused of spying. He returned to the Foreign Office in 1933, and held office as head of the League of Nations section until 1937 and of the Central Department from 1937 to 1939. During the 1930s he was an adviser to the government at the major international meetings, and met Mussolini, Hitler and Stalin. He was a tacit opponent of appeasement, but always stayed loyal to the government. From 1939 to 1943 he was assistant under-secretary of state for Europe. During the late-1930s, Strang was a member of the Anglo-German Fellowship, which was pro-Nazi. Strang became of interest to an undercover MI5 agent, Eric Roberts, who was operating under the pretence of working for the Gestapo, with the intention of identifying potential fifth columnists. In 1943, he reported on one of the diplomat's female friends who was possibly his lover. Unaware of her connections, Strang had told her "that he personally hated the Jews and regarded the Bolsheviks and the Jews as the two greatest enemies of all that is decent". Roberts reported further comments made six days later to the same woman: "Strang alleged that the Bethnal Green tube disaster was caused by a Jewish pickpocket gang, the ringleader of which netted £200". Ultimate responsibility for the Bethnal Green tube disaster in March 1943 was placed on the negligence of the authorities rather than any individuals. Strang was present at the major conferences between the Allied leaders during the Second World War. In 1943 Strang was appointed the British representative on the European Advisory Commission, with the rank of ambassador. The commission was established by the Allies to study the possible post-war political problems in Europe and make recommendation but was dissolved at the Potsdam Conference. In June 1945, Strang became political adviser to the Commander-in-Chief of British forces in Germany, Bernard Montgomery. Strang again returned to the Foreign Office in 1947 and served as Permanent Under-Secretary of State for the German section from 1947 to 1949 and as Permanent Under-Secretary of State for Foreign Affairs from 1949 to 1953. The six years Strang served as Permanent Under-Secretary of State saw the gradual recovery of Europe through the Marshall Plan, the establishment of the Western European Union and NATO and the breaking of the Berlin blockade. He retired from the Foreign Office in December 1953 due to ill health, following an incident where he fainted during Queen Elizabeth II's coronation ceremony at Westminster Abbey.
Because of standing up for Baltic Sea island of Fehmarn so that it did not become part of the Soviet occupation zone, as was Stalin's wish, William Strang is highly revered on the island, although he never visited it during his lifetime. Lord Strang died at the age of 85.
