Deedes fought with the British Army in the Second World War, being based initially at Shrapnel Barracks in Woolwich as an Officer in the 2nd Battalion, Queen's Westminsters, one of the Territorial units of the King's Royal Rifle Corps. He gained the Military Cross near Hengelo, The Netherlands in April 1945. He was also the only officer to serve in 12th King's Royal Rifle Corps (2nd Queen's Westminsters) for the duration of the war. His battalion served as the motorised battalion of 8th Armoured Brigade in the North-west Europe campaign.