Upon the outbreak of World War I, "Implacable", along with the squadron was assigned to the Channel Fleet. After operations with the Dover Patrol, she served in the Dardanelles Campaign in support of the Allied landings at Gallipoli. She participated in the Landing at Cape Helles on 25–26 April and supported ANZAC forces ashore over the course of the following month. In late May 1915, she was withdrawn to reinforce the Italian fleet at the southern end of the Adriatic Sea after Italy joined the war on the side of the Allies. She remained in the Mediterranean until June 1917, apart from a brief return to Britain in March and April 1916 for a refit. After returning to England, she was laid up until March 1918, when she was converted for use as a depot ship for the Northern Patrol. After the war, she was decommissioned and eventually sold for scrap in 1921.