On 8 March 1940, Porritt was commissioned a lieutenant in the Royal Army Medical Corps with the service number 125494. He served in France until the evacuation from Dunkirk, then in Egypt, operating on seriously wounded soldiers from the North African campaign, and later landing in Normandy on D-Day. A war-substantive major by February 1943, he was granted the temporary rank of lieutenant colonel and the acting rank of colonel later that year; on 18 December 1943, he was appointed a consultant surgeon with the local rank of brigadier. He relinquished this position and his brigadier's rank on 1 September 1945, by which time he was a war-substantive lieutenant colonel. He ended his military career in September 1956, with the honorary rank of colonel in the Territorial Army.