After the start of the Second World War in 1939 Benson became a second lieutenant in the Grenadier Guards thanks to the influence of his uncle D'Arcy Cooper and Viscount Trenchard. Benson spent a period guarding Windsor Castle, and while there wrote a report on improving the ways in which the soldiers communicated. In 1942 he was promoted to Major and transferred to the Special Operations Executive, where he stayed for two years until he was promoted to Colonel and transferred to the Ministry of Supply to help reorganise munitions factories. Benson became Director of Operations (accounts), putting him in charge of 100,000 people, and by the time he left the army in 1945 he was an acting Brigadier. After leaving the army he was made a Commander of the Order of the British Empire in recognition of the work he had done.
