In 1892, Sinclair was elected a Liberal Member of Parliament for Dunbartonshire, a seat he held until 1895, and returned to the Commons representing Forfarshire from 1897 to 1909 [REF].
He was appointed a Privy Councillor on 11 December 1905[REF] and was He was created Baron Pentland, of Lyth in the County of Caithness on 15 February 1909 [REF].