Alexander (also called Alastair) Denniston was born 1 December 1881. In 1917 he married Dorothy Mary Gilliat and they had two children Robin and Margaret. His first job was Assistant Master at Merchiston Castle School, 1906-1909. Denniston was a member of the Scottish Olympic Hockey team in 1908. He then taught German at the Royal Naval College at Osborne, 1909-1914. He joined the Admiralty Naval Intelligence Division (NID) a few weeks after the outbreak of World War I, working out of Room 40 (or 40 OB and later called ID 25) of the Old Building of the Admiralty. Denniston soon became second in command of Room 40. After the end of the war he became head of Room 40's successor, the Government Code and Cipher School (GC&CS) and he retired in 1945. His wife also worked in Room 40. Denniston became a Commander RNVR in 1917. Denniston died 1 January 1961.