During World War I, as a general staff officer with the American Expeditionary Force, Heintzelman was responsible for planning the St. Mihiel Offensive. He also served with the French in the Chemin des Dames offensive in October 1917, and in winter operations in northern Italy with the French Tenth Army. He concluded his tour in France as Chief of Staff for both IV Corps and later Second Army. From 1921 to 1929, he held numerous command and staff positions including command of the 22nd Infantry Brigade, commander of the Eastern Defenses of New York, and Assistant Chief of Staff for War Plans. In 1929, he again returned to Fort Leavenworth to serve as the Commandant of the Command and General Staff College and was promoted to Major General in 1931. As Commandant, Heintzelman was influential in updating the curriculum and actively participated in the classroom.