Sinclair studied at the Royal Academy, Woolwich and in 1919 was commissioned in the Royal Artillery. A successful officer Sinclair became Deputy Director of Military Operations during the Second World War.
In 1953 Sinclair replaced Major General Stewart Menzies as Director-General of MI6. Three years later Commander Lionel Crabb, an underwater sabotage expert, disappeared in April 1956 on a secret mission to investigate the Russian cruiser Ordkhonikidze. This created a diplomatic row as the ship had brought over Nikita Khrushchev and Nikolai Bulganin on a goodwill mission to Britain. Sir Anthony Eden was furious and as a result forced Sinclair to resign. He was replaced by Sir Dick White, the former head of MI6.
Sir John Sinclair died in 1977.