Leslie Brown was born on 7 July 1920 in Toppenish, Washington. He graduated from high school in 1938 together with his buddy and future marine general, Jay W. Hubbard and attended Compton College in California prior to enlisting in the U.S. Marine Corps in 1940. He was subsequently attached to the Marine detachment aboard the USS "Mississippi" and spent next three years with sea duties. Brown reached the NCO's rank and was transferred to 2nd Marine Regiment at New Zealand. Brown subsequently received Field promotion to the rank of second lieutenant in June 1943 and was attached as platoon leader to the Weapons Company of 6th Marine Regiment under Colonel Maurice G. Holmes. His regiment then took part within 2nd Marine Division in the Amphibious Assault on Tarawa Atoll in the Gilbert Islands at the end of November 1943 and Brown quickly reached the rank of first lieutenant.