Simkin was called up for military service on 15 July 1940 and joined the Royal Artillery. He was a member of a team operating an anti-aircraft battery during the Battle of Britain. Later he served at Biggin Hill, Rochford and Dover. He was wounded in 1944 and as a result missed taking part in the D-Day landings.
After the Second World War he worked at the Every-Ready factory in Edmonton. John Simkin died in road accident on 16th October, 1956.