She served in the First World War, initially as part of the 2nd Light Cruiser squadron in the Mediterranean. In August 1914 Weymouth was detached to sail into the Indian Ocean to hunt for the German cruiser SMS Emden, which was raiding allied shipping in the area. In February 1915 she was operating off the East African coast as part of operations against another commerce raider, the SMS Königsberg, eventually trapping her in the Rufiji River until she could be sunk.
Weymouth returned to the Mediterranean, and in December 1915 was operating in the Adriatic. Whilst here she was involved in the Battle off Durazzo on December 28-29. In 1916 she returned to home waters and was assigned to the 6th Light Cruiser Squadron of the Grand Fleet. In 1917 she was reassigned to the Mediterranean as part of the 8th Cruiser Squadron operating out of Brindisi. Whilst serving here, she was damaged by a torpedo from the Austrian submarine U 28 on 2 October 1918. She was repaired and survived the war. She was sold on 2 October 1928 to Hughes Bolckow, of Blyth.