Porritt was in the Officer Training Corps at Marlborough and was commissioned in the Lancashire Fusiliers regiment of the British Army in 1931 where he served in a Territorial Army unit. During World War II he served as a captain in the 1/5th Bn of the Lancashire Fusiliers and was killed in action in 1940 aged 29. He was killed in the town of Seclin, near Lille during the retreat to Dunkirk one day after the order to evacuate the Allied force from France in the wake of the German offensive. The British Army were retreating towards the Channel coast at the time trying to establish a defensive line behind which the evacuation was being progressed.