He joined the British Army in 1914, following the start of the First World War. He was mentioned in despatches and promoted to the rank of Major. In 1916, he was awarded the Military Cross (MC) while serving on the Western Front as a brigade major to the 124th Brigade of the 41st Division. During the war, Cunliffe-Lister spent time with Winston Churchill at his advanced HQ Lawrence Farm. They later worked together in the Stanley Baldwin ministries of the 1920s, when Cunliffe-Lister served as a minister of state. In 1917 he was appointed joint secretary to the Minister of National Service. He was noticed by David Lloyd George, who recruited the young man to be chairman of the Labour sub-committee of the war cabinet in Downing Street. At the end of the war, he stood as a Conservative candidate in the Coupon election of 1918.