McLennan participated in combat operations in Korea as the S-4 Officer, 3rd Battalion, 5th Marine Regiment, 1st Marine Division from January to July 1953. He later served as Commanding Officer, Company I, 3rd Battalion, 5th Marines and as Assistant S-3 Officer, 5th Marines. He integrated into the regular Marine Corps in March 1953. He returned to the United States in February 1954, and served as Assistant Division Embarkation Officer, 2nd Marine Division at Camp Lejeune, North Carolina, until June 1955, when he was transferred to Salt Lake City, Utah, as Inspector-Instructor, 21st Rifle Company, USMCR. He was promoted to major in July 1956, and, in August 1957, he attended the Amphibious Warfare School Junior Course at Quantico, graduating in July 1958.