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 survived the sinking and landed on Guadalcanal with survivors and served there for three months with the small naval unit attached to the First Marine Division For his three-month service at Guadalcanal, Mustin was decorated with the Navy Commendation Medal with Combat "V".