Dreyer moved to the Admiralty with Jellicoe as the Assistant Director of the Anti-Submarine Division. In March 1917 he was appointed DNO, where he formed a committee to design and produce a new type of armour-piercing shell, as the existing type had proved woefully unreliable. He was appointed to the Naval Staff as Director of Naval Artillery and Torpedoes in 1918. Following the Armistice he was appointed commodore, 2nd class and served as Chief of Staff to Admiral Jellicoe on his Naval Mission to India and the Dominions on HMS "New Zealand", between 1919 and 1920.