Stanley Graves Benner was born on 5 July 1916 in Arlington, Massachusetts. He lived in Boston, Massachusetts, until 1940. Enlisting in the United States Marine Corps Reserve on 21 August 1940, he reported for active duty at Marine Corps Base Quantico, on 8 November that same year. After training at the Marine Corps' recruit depot at Marine Corps Recruit Depot Parris Island, he arrived at Guantanamo Bay Naval Base, on 21 January 1941. While there, he participated in maneuvers on the Puerto Rican Island of Culebra. Transferred to the Marine Corps Base at Parris Island, S.C., on 12 April, he served there—receiving a promotion to private first class on 26 May, until shifting duty station to what would later become Camp Lejeune at New River, N.C., on 28 September. Benner was promoted to corporal on 11 October 1941 and to Sergeant on 1 April 1942.