Archibald Sinclair at Chartwell

Signatures in Churchill's visitors book at Chartwell, a National Trust property in Kent

Archibald Henry Macdonald Sinclair, 1st Viscount Thurso, KT, CMG, PC (1890–1970), was a British politician and leader of the Liberal Party.

Born in Chelsea, London, Sinclair was the son of a Scottish father Clarence Granville Sinclair and an American mother, Mabel Sands. Sinclair's mother died a few days after his birth, and five years later his father died, leaving him to grow up moving between the houses of various relatives.

Educated at Eton and at Sandhurst, Sinclair entered the army in 1910 in the 2nd Life Guards. On the death of his grandfather in 1912 he succeeded to the baronetcy and with it the ownership of some 100,000 acres in Caithness.

The deepest friendship Sinclair made at this time was with Churchill as they discovered that they had a vast amount in common. Both felt that circumstances had deprived them of full parental affection, though both had been sustained by a devoted governess. Both had American mothers and swashbuckling Yankee grandfathers. Both were by training cavalry officers with a shared enthusiasm for polo and flying. It was even the case that both had a slight speech impediment.

Sinclair served on the western front throughout the First World War. In January 1916 Churchill, whose career had been ruined by the Gallipoli campaign, took charge on the western front of the 6th Royal Scots Fusiliers and for four months Sinclair served as his second in command. In 1918 Sinclair met and married Marigold Forbes; they had four children.

After the war Sinclair remained as aide to Churchill, serving as his personal military secretary at the War Office (1919–1921), and his private secretary at the Colonial Office (1921–1922). In 1922 he entered parliament as MP for Caithness and Sutherland, taking his stand as a Liberal supporting the Lloyd George (and thus Churchill) wing of the party.

In the House of Commons he gained a reputation as a skilful opposition speaker, while playing a part in the comprehensive overhaul of Liberal policy which Lloyd George had initiated and in 1935 Sinclair became the party's leader.

When Churchill formed an all-party coalition government in 1940, Sinclair became Secretary of State for Air. However, he did not sit in the small War Cabinet, though he was invited to attend meetings discussing any political matter. He remained a minister until May 1945 when the coalition ended. In the 1945 general election, he narrowly lost his seat.

In 1952, he accepted elevation to the House of Lords as 1st Viscount Thurso of Ulbster in the County of Caithness.