Sinclair first cut his political teeth on the London County Council as Progressive Councillor for Finsbury East from 1889 to 92 and was a founder member of the London Playing Fields Society. In parliament, Sinclair acted as Parliamentary Secretary to Sir Henry Campbell-Bannerman and the two men formed a close political and personal friendship. When Campbell Bannerman became Prime Minister, Sinclair was appointed Secretary of State for Scotland, serving in the post until 1912 when he was appointed Governor of Madras. Sinclair returned to the UK in 1919 and retired from active public life.