On 12 March 1918, as the Germans prepared to launch their Spring Offensive, they laid down a heavy mustard gas barrage on Dempsey's battalion, which was now at La Vacquerie with Dempsey commanding D Company. Dempsey, along with 10 officers and 250 other ranks, was gassed and later evacuated to England, where he had a lung removed. He returned to the battalion on 6 July, where, with the tide of the war having turned, the 1st Royal Berkshires took part in the Hundred Days Offensive until the war ended on 11 November 1918. Dempsey served as adjutant again from 5 October to 4 November. Dempsey was mentioned in despatches on 8 November 1918, and awarded the Military Cross (MC), which was gazetted in the King's Birthday Honours list on 3 June 1919.