Career
Between 1919 and 1921, he was Director of British Naval Intelligence, and helped to set up the Secret Intelligence Service (SIS, commonly MI6) before the Second World War. Sinclair educated at Stubbington House School, he joined the Royal Navy at 13 in 1886 and entered the Naval Intelligence Division at 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.
Beginning in 1919 he attempted to absorb the counter-intelligence service MI5 into the SIS to strengthen Britain"s efforts against Bolshevism, an idea was finally rejected in 1925. The SIS remained small and under-funded during the inter-war years.
By 1936 Sinclair realized 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 Doctorate, dedicated to sabotage. In spring of 1938, using 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.
Sinclair became seriously ill with cancer, causing Alexander Cadogan to note on 19 October 1939, that he was "going downhill". On 29 October, he underwent an operation for his cancer and died on 4 November 1939, aged 66, five days before the Venlo incident.