A graduate (1917) of the U.S. Military Academy at West Point, N.Y., Clark served overseas in World War I. Early in 1942 he became chief of staff of army ground forces. Later that year, as deputy commander in chief to General Dwight D. Eisenhower, he executed delicate and demanding assignments in connection with the Allied invasion of North Africa, including a dramatic submarine trip to Algeria for a secret meeting with French officers.

Clark's responsibilities were considerably enlarged when he was appointed commander of the American 5th Army, which effected a major landing at Salerno (September 1943) aimed at wresting the Italian peninsula from Axis control. Clark received the surrender of the Italian fleet and the government of Marshal Pietro Badoglio the same month; his march into Rome (June 4, 1944) marked the fall of the first enemy capital. In December he was appointed commander of the 15th Army Group and finally received the surrender of the stubborn German forces in the north of Italy on May 2, 1945.

After hostilities ended in Europe, Clark assumed command of U.S. troops in Austria before returning home to command the 6th Army and later the army field forces. In May 1952, during the Korean War, he was given command of all United Nations troops in Korea, holding that post until after an armistice was signed (July 1953); he retired from the army the same year. Clark served as president of The Citadel, a military college in Charleston, S.C., from 1954 to 1966. He wrote Calculated Risk (1950), an account of his experience of World War II, and From the Danube to the Yalu (1954), his perspective on the Korean War.