She was still assigned to the 3rd Battle Squadron of the Grand Fleet in Scapa Flow at the start of World War I. On 15 December the ship was badly damaged due to severe weather in the Pentland Firth as she sortied to intercept German ships bombarding ports in Yorkshire and had to return to port for repairs. "Blanche" was transferred to the 4th Battle Squadron, joining her sister ship, , in January 1916. On 28 February, she was one of three cruisers dispatched to patrol off the Norwegian coast during the hunt for the German raider SMS "Greif", although she did not come into contact with the German ship before she was sunk. Captain John Casement relieved Hyde on 21 May. During the Battle of Jutland, she was assigned to a position at the rear of the squadron during the battle and did not fire her guns. By January 1917, "Blanche" was detached from the Grand Fleet, presumably to be converted into a minelayer and Casement was relieved by Captain The Honourable Reginald Plunkett-Ernle-Erle-Drax on 15 January. The ship had been assigned to the 5th Battle Squadron by April. "Blanche" laid mines at the entrance to the Kattegat on the nights of 18/19 and 24/25 February 1918, part of her total of 1,238 mines laid during 16 sorties during the war. Captain Francis Buller assumed command in lieu of Plunkett-Ernle-Erle-Drax on 5 April 1918. He, in turn, was relieved by Captain Charles Wrightson on 7 January 1919. The ship was still assigned to the 5th Battle Squadron in February, but had been assigned to the Nore Reserve by 1 May, together with "Blonde". The sisters were listed for sale by 18 March 1920 and "Blanche" was sold to Fryer for scrap on 27 July 1921 and broken up at Sunderland.