The son of millionaire Andrew B. Hammond of San Francisco, California, socialite Leonard Coombes Hammond attended Harvard in 1901 and 1902. From November 1915 until November 1916 he was a driver in the Norton-Harjes Ambulance Corps on the Western Front. Upon his return to San Francisco, he entered officer’s training at the Presidio in May 1917 and was commissioned First Lieutenant on 15 August 1917. As an observer, Lieutenant Hammond was credited with 6 shared aerial victories, becoming an ACE in World War I.