[3] Commissioned into service in the Royal Navy in June 1915, Cleopatra was assigned to the 5th Light Cruiser Squadron in Harwich Force, which operated in the North Sea to guard the eastern approaches to the Strait of Dover and English Channel.

[3] In August 1915, she took part in the hunt in the North Sea for the Imperial German Navy auxiliary cruiser SMS Meteor.

In February 1916, she replaced the recently lost light cruiser HMS Arethusa as flagship for Harwich Force's commander, Commodore Reginald Tyrwhitt.

She turned toward G 194 and rammed her, cutting the destroyer in half and sinking her immediately, but the maneuver took Cleopatra across the bows of the light cruiser HMS Undaunted, and the two cruisers collided; Cleopatra returned to base with the force despite the damage she suffered in the two collisions, but Undaunted was so badly damaged that it took her four days to reach port.

She again recommissioned in January 1925 and was assigned to the 2nd Light Cruiser Squadron in the Atlantic Fleet, serving until decommissioned again in December 1926 and placed under dockyard control.