 He served in the Second World War as a Lieutenant-Colonel in the British Army, was twice Mentioned in Despatches and awarded the Croix de Guerre.After the war, he served in the Conservative administrations of Winston Churchill and Anthony Eden as a government whip from 1952 to 1954 and as Under-Secretary of State for the Home Department from 1954 to 1957.When Harold Macmillan became Prime Minister in January 1957, Mancroft was appointed Parliamentary Secretary to the Minister of Defence, Duncan Sandys, a post he held until June the same year, and was then Minister without Portfolio from 1957 to 1958.