Born in Pennsylvania, Marshall graduated from the Virginia Military Institute (VMI) in 1901. Marshall received his commission as a second lieutenant of Infantry in February 1902 and immediately went to the Philippines. He served in the United States and overseas in positions of increasing rank and responsibility, including platoon leader and company commander in the Philippines during the Philippine–American War. He was the Honor Graduate of his Infantry-Cavalry School Course in 1907, and graduated first in his 1908 Army Staff College class. In 1916 Marshall was assigned as aide-de-camp to J. Franklin Bell, the commander of the Western Department. After the nation entered World War I in 1917, Marshall served with Bell who commanded the Department of the East. He was assigned to the staff of the 1st Division, and assisted with the organization's mobilization and training in the United States, as well as planning of its combat operations in France. Subsequently, assigned to the staff of the American Expeditionary Forces headquarters, he was a key planner of American operations including the Meuse-Argonne Offensive.