Politician and landowner. Educated at Eton College and Sandhurst, Sinclair joined the Life Guards in 1910 and went on to serve in the First World War, for a time with Winston Churchill (1874 - 1965). The pair became close friends and later political allies. In 1912 Sinclair succeeded his grandfather to become the 4th Baronet of Ulbster, with substantial estates in Caithness including the now-ruined Thurso Castle.
In 1922, he was elected as Liberal Member of Parliament for Caithness and Sutherland, and served until 1945. He became Chief Whip of the Liberals in 1930. Between 1935 and 1945 served as Leader of a party which had become marginalised in British politics and he fought to restore their relevance. He served in two National unity governments as Secretary of State for Scotland (1931-32) and Secretary of State for Air (1940-45).
Sinclair served as Rector of the University of Glasgow (1938-45). He was created Viscount Thurso in 1952.
His grandson, John Thurso (b.1953) now represents Caithness, Sutherland and Easter Ross in the Westminster Parliament, while his granddaughter, Veronica Linklater (b.1943), supports the Liberal-Democrats in the House of Lords.
