He was educated at Eton College, New College, Oxford and Edinburgh University.
He was Captain of Boats at Edinburgh University Boat Club from 1946-1947, winning a Green in 1946, and a Blue in 1947. He succeeded his father to the title of Viscount Thurso in 1970. Sinclair served in the Royal Air Force from 1941-1946.
He was a Flight Lieutenant in 684 Squadron, 540 Squadron and he commanded the Edinburgh University Air Squadron in 1946.
Sinclair was elected as a Liberal to Caithness County Council in 1949. In 1957 he was elected to Thurso Town Council.
He served on both the county and town councils until 1961 and then again from 1965 until 1973. He was Liberal candidate for the East Aberdeenshire division at the 1966 General Election.
The constituency had been safe Conservative in 1964 with the Liberals second.
He did not stand for parliament again. Electoral record
He was an active supporter of the Boys" Brigade. In 1959 he was appointed a Justice of the peace.
In 1973 he was appointed Lord Lieutenant of Caithness.
He was a member of the Scottish Liberal Party Executive.