Bearss enlisted in the United States Marine Corps in 1941. During World War II, he served in the 3rd Marine Raider Battalion; he fought in the Guadalcanal and New Britain campaigns with the 1st Marine Division. In 1943, Bearss caught malaria in the South Pacific, and was sent to New Zealand to recover. On 2 January 1944 with the 3rd Battalion, 7th Marines at the Battle of Cape Gloucester, Bearss was hit by Imperial Japanese Army machine-gun fire that broke both of his arms and injured his heel and buttocks; after spending the next 26 months in hospital, he left the Marines in March 1946 with the rank of corporal.