Douglas Sealy was born in Matchan, Bengal, India on 23 November 1891, the son of Edmund Thomas Sealy and Annie Emily Sealy. He joined the Royal Navy as a Cadet on 15 September 1904. After time in the Battleship HMS DREADNOUGHT and then HMS PATROL, he joined submarines as a Sub Lieutenant in June 1913.

After promotion to Lieutenant in April 1914, he became First Lieutenant of HMS C15 and then, in October 1914, of D3. Sealy next commanded HMS C27, which, in August 1916, was sent into the Baltic, as a result of which he was awarded the DSC ‘in recognition of his services in submarines in enemy waters’. During his time in the Baltic, he made an attack on the German Battleship KONIG in October 1917, but his torpedoes ran under as he had fired from too short a range. He also attacked the German Troop Transport INDIANOLA, causing damage, but C27 was also damaged in a counterattack.

He next took command of HMS E48 in February 1918 and, in April of that year, was awarded a Bar to his DSC for his service in the Baltic. After a spell in surface ships, Sealy returned to submarines in command of HMS H42 in December 1921. Sealy died when H42 was sunk in a collision with HMS VERSATILE off Gibraltar on 23 March 1922. There were no survivors.