sinclair was born in kingston vale, the eldest son of the leader of the liberal party, archibald sinclair, 1st viscount thurso, and marigold forbes, the older daughter of lady angela and colonel james stewart forbes. he was educated at eton college, new college, oxford, and edinburgh university, where he was captain of boats at the university boat club from 1946 to 1947, winning a green in 1946, and a blue in 1947. in 1952 he married margaret beaumont (1918-2017). they had two sons and one daughter. he succeeded his father to the title of viscount thurso in 1970. he was succeeded by his son john upon his death in 1995.