Arundell served in the Territorial Army as a second lieutenant on the General List from 1929[1] to 1931.[2] He joined the 4th Battalion of the Wiltshire Regiment as a lieutenant in 1934,[3] and was promoted to captain in 1939.[4] With the 2nd Battalion the Wiltshire Regiment, he was part of the British Expeditionary Force and in 1940, during the Battle of France, was taken prisoner-of-war by the Germans. He escaped, but was recaptured and was transferred to Oflag IV-C at Colditz Castle. There he had a habit of exercising in the snow and, perhaps as a result, contracted tuberculosis. In accordance with the Geneva Conventions, he was repatriated and died in hospital in Chester. He was buried in the Chapel of New Wardour Castle. He was unmarried, and the barony became extinct upon his death.