During World War I, Marshall had roles as a planner of both training and operations. In the summer of 1917, he was assigned as assistant chief of staff for operations on the staff of the 1st Division. After overseeing the division's mobilization and organization in Texas, he departed for France with the division staff in mid-1917. On the long ocean voyage, his roommate was the division's assistant chief of staff for training, Lesley J. McNair; the two formed a personal and professional bond that they maintained for the rest of their careers.