Appointed an ensign in the United States Naval Reserve, to rank from August 29, 1940, he attended the Navy Supply Corps School, Harvard Graduate School of Business Administration.
He was then assigned to the New York Navy Yard at Brooklyn, New York, as a supply officer, reporting there on March 10, 1942. Ensign Cox reported to the light cruiser United States Ship Juneau (CL-52) on May 29, 1942 to serve as an assistant disbursing and supply officer He was reported missing in action when a Japanese submarine torpedoed and sank Juneau off Guadalcanal during the Naval Battle of Guadalcanal on November 13, 1942.
The United States. Navy destroyer escort United States Ship Cox (Delaware-774) was named for Ensign Cox.
Her construction was cancelled in 1944.