Son of a Scottish father and an American mother, Sinclair served on the western front during World War I and was Winston Churchill’s second-in-command and later his private secretary when Churchill returned to the cabinet in 1919. At the 1922 general election Sinclair was elected MP for Caithness and Sutherland During his parliamentary career Sinclair held cabinet office twice, as Secretary of State for Scotland in the National Government of 1931 and as Secretary of State for Air during the wartime coalition of 1940-45. Sinclair narrowly lost his seat in the 1945 election coming third but only 61 votes behind the Tory winner. After an attempt to return to the Commons in 1950, he accepted a peerage as Viscount Thurso. The party hoped that Sinclair would lead the Liberal peers but a series of strokes forced Sinclair to retire from active politics.