During World War II, he commanded the divisional artillery. He led the artillery overseas during the Allied invasion of Sicily in July–August 1943, where he earned the first of two Distinguished Service Crosses. He continued to lead the artillery in the Allied invasion of Italy in September, and in the early stages of the Italian Campaign, before eventually being sent to England. During the Battle of Normandy in August 1944, McLain briefly took command of the 90th Infantry Division, which was having numerous command problems. McLain transformed the 90th into a first-class fighting formation, and led it across France in the Allied advance from Paris to the Rhine and in many numerous battles on the Western Front. In October he then assumed command of the XIX Corps, remaining its commander for the rest of the war. He was the only National Guardsman to command a corps in combat during World War II.