HMS E9, commanded by Lieutenant Commander Max Horton, scored the first offensive success of the First World War by a British submarine. On 13 September 1914, Horton sighted the German cruiser Hela near Heligoland, closed to 600 yards and fired two torpedoes, one of which struck amidships. Hela sank and E9 was hunted for the remainder of the day but successfully escaped.

During her next patrol in the Heligoland Bight, E9 sank the destroyer S116. On 17 December 1915, HMS E9 sank the German cruiser Bremen and V191 in the East Baltic.

E9 was deliberately scuttled on 3 March 1918 in the Baltic off Helsingfors to prevent capture by the German Army.