Sinclair was educated at Stubbington House School and joined the Royal Navy at 13 in 1886 and entered the Naval Intelligence Divisionat the beginning of the First World War. He became Director of Naval Intelligence in February 1919 and Chief of the Submarine Service in 1921. He became the second director of SIS in 1923. He was promoted vice-admiral on 3 March 1926 and full admiral on 15 May 1930. Beginning in 1919 he attempted to absorb the counterintelligence service MI5 into the SIS to strengthen Britain's efforts against Bolshevism, an idea that was finally rejected in 1925. The SIS remained small and underfunded during the interwar years. By 1936, Sinclair realised that the Gestapo had penetrated several SIS stations and Claude Dansey, who had been removed from his station in Rome, set up Z Organization, intended to work independently of the compromised SIS. In 1938, with a second war looming, Sinclair set up Section D, dedicated to sabotage. In spring 1938, using £6,000 of his own money, he bought Bletchley Park to be a wartime intelligence station. Sinclair was asked in December 1938 to prepare a dossier on Adolf Hitler, for the attention of Lord Halifax, the Foreign Secretary, and Neville Chamberlain, the Prime Minister. In the dossier, which was received poorly by Sir George Mounsey, the Foreign Office assistant undersecretary, who believed that it did not gel with Britain's policy of appeasement, Sinclair described Hitler as possessing the characteristics of "fanaticism, mysticism, ruthlessness, cunning, vanity, moods of exaltation and depression, fits of bitter and self-righteous resentment; and what can only be termed a streak of madness; but with it all there is a great tenacity of purpose, which has often been combined with extraordinary clarity of vision". Sinclair became seriously ill with cancer, causing Alexander Cadogan to note on 19 October 1939, that he was "going downhill". On 29 October, Sinclair underwent an operation for his cancer and died on 4 November 1939, aged 66, five days before the Venlo incident.
