He took part in developing the Navy's first lead-computing anti-aircraft gun sight, which proved of major importance in the air-sea actions of World War II, and he served on the cruiser USS Atlanta during the naval battle of Guadalcanal.

His ship was lost during that action, and with other survivors he landed on Guadalcanal and served ashore with a naval unit attached to the First Marine Division.

Additionally, his great grandfather, Captain Arthur Sinclair, USN, commanded one of Commodore Perry's ships in the opening of Japan.

At the outbreak of World War II, Mustin was assistant gunnery officer in the cruiser Atlanta (CL 51) and was aboard when she was sunk during the Battle for Guadalcanal on the night of 13 November 1942.

He next served as gunnery, radar, and CIC officer on the staff of Commander Battleship Squadron Two, Vice Admiral Willis A. Lee Jr., USN, and in the summer of 1945 was with Admiral Lee in establishing the Operational Development Force, as gunnery, radar, and CIC officer.

[5] After the war, Mustin was ordered to the Navy Department, Washington, D.C., for duty as head of the Fire Control Branch, Research Division, Bureau of Ordnance.

In May 1967 he became commander of Amphibious Force, US Atlantic Fleet and in July 1968 assumed duty as director of the Defense Atomic Support Agency, Washington, D.C.

[5][2][8] Following his retirement from the Navy, Mustin was a member of the U.S. Olympic Committee for shooting sports and served as president of the National Rifle Association of America 1977–1979.

He was buried beside his wife, Emily Morton Mustin (1914-1989), at United States Naval Academy Cemetery at Annapolis, Maryland.