Forbes served in the First World War as Executive Officer in the battleship in the Mediterranean Fleet from November 1914 and saw action in the Dardanelles campaign in April 1915. He became Flag Commander to Admiral Sir John Jellicoe, Commander-in-Chief of the Grand Fleet, in the battleship in October 1915 and saw action at the Battle of Jutland in May 1916 for which he was awarded the Distinguished Service Order on 15 September 1916. He moved on to become Flag Commander to Admiral Sir Charles Madden, Second-in-Command of the Grand Fleet, in the battleship in February 1917. He was awarded the Order of Saint Stanislaus, 2nd Class with Swords for his action during the Battle of Jutland on 5 June 1917. Promoted to captain on 30 June 1917, he was given command of the cruiser in the Grand Fleet in July 1917 and was present at the surrender of the German fleet in November 1918.