Clark graduated from West Point in 1917. He had gained an early appointment to the military academy, but lost time from illnesses. He was appointed to the rank of captain in the infantry in 1917 and served in France during World War I in the U.S. 11th Infantry, where he was wounded.
Between the wars, Clark served as a deputy commander of the Civilian Conservation Corps district in Omaha, Nebraska. He attended the Command and General Staff School in 1935 and the Army War College in 1937. Clark had retained his World War I rank of Captain following the armistice and was promoted to Major in 1932. Major Clark was promoted to Lieutenant Colonel in 1940, and in August 1941 was promoted two grades to temporary Brigadier General, and received further promotion to temporary Major General less than a year later.
After his negotiations with the Vichy French at Cherchell, Major General Clark was promoted to temporary Lieutenant-General in November 1942 and was subsequently given command of the Fifth U.S. Army shortly before the invasion of Italy (Operation Avalanche) in September 1943. The near-failure of the landings at Salerno has been blamed on Clark's poor planning. Clark commanded the bombing destruction of the Abbey of Monte Cassino during the battle of Monte Cassino, February 15, 1944. Clark's conduct of operations remains controversial, particularly his actions during the Battle of the Winter Line, when, ignoring orders from his Army Group Commander, the British Harold Alexander, he sent his tired units towards Rome, which was captured on 4 June 1944 (two days before the Normandy landings), rather than exploiting the gap in the German positions to entrap and capture German units. In December 1944 Clark succeeded Harold Alexander in command of the 15th Army Group, putting him in command of all Allied ground troops in Italy, which resembled more a United Nations coalition - a hodgepodge of diverse cultures with conflicting interests.
He was promoted to general on March 10, 1945 and at the war's end Clark was Commander of Allied Forces in Italy and, later, U.S. High Commissioner of Austria. He served as deputy to the U.S. secretary of state in 1947, and attended the negotiations for an Austrian treaty with the Council of Foreign Ministers in London and Moscow. In June 1947, Clark returned home and assumed command of the Sixth Army, headquartered at the Presidio in San Francisco, and two years later was named chief of Army Field Forces.
After retiring from the Army, General Clark served (1954 to 1966) as president of The Citadel, The Military College of South Carolina, in Charleston, South Carolina. He wrote two volumes of memoirs: Calculated Risk (1950) and From the Danube to the Yalu (1954).
Clark is buried at The Citadel.
An Interstate spur (I-526) connecting different suburbs in the Charleston metropolitan area and a bridge in Washington state connecting Camano Island with the mainland bear his name.