Lemnitzer was a graduate of the U.S. Military Academy, West Point, N.Y. (1920), the Command and General Staff School, Fort Leavenworth, Kan. (1936), and the Army War College, Carlisle, Pa. (1940). He was instrumental in planning the invasion of North Africa in World War II and once was nearly apprehended by Vichy police in Algeria (1942). A skilled diplomat, he participated in secret negotiations with Italy's Premier Pietro Badoglio (Badoglio, Pietro) that led to Italy's surrender to the Allies (1943), and he also conducted secret talks with the German High Command that led to the surrender of German armies in Italy and southern Austria (1945).

Qualifying as a paratrooper at the age of 51, Lemnitzer took command of infantry troops in Korea (1951–52). After serving as commander of all U.S. and UN forces in Korea and Japan, he returned to Washington, D.C., to become army vice chief of staff under General Maxwell D. Taylor (1957). After Taylor retired, Lemnitzer succeeded him and was named chairman of the Joint Chiefs of Staff (1960), a post he held until he was appointed supreme allied commander in Europe (1963). Lemnitzer retired from active duty in 1969 and received the Presidential Medal of Freedom in 1987.