James Sinclair was born on 12 February 1895 at Edinburgh, the son of John and Rose Ann Sinclair.
He joined the Royal Navy as a Boy 2nd Class on 15 April 1912 at HMS GANGES II (Shotley), giving his previous occupation as warehouseman. From July 1912 to May 1914 he served in the armoured cruiser HMS DEVONSHIRE, being rated Ordinary Seaman on his 18th birthday, and Able Seaman a year later. After a short period in the pre-dreadnought battleship HMS LONDON, he joined HMS DEFIANCE, the Devonport torpedo school where he qualified as a Torpedoman. In March 1915 he joined the battleship HMS BENBOW where he saw action on 31 May 1916 at the Battle of Jutland. In September 1916 he returned to the naval barracks at Devonport.
He joined HMS DOLPHIN on 4 November 1916, but died in the RN Hospital at Haslar on 29 November from a cerebral abscess. He is buried at Haslar Royal Naval Cemetery.