Following the United States' entry into World War I, Key was promoted to the rank of major and embarked for France. He was attached to the headquarters, 7th Division under Brigadier General Charles H. Barth and served as divisional quartermaster during Champagne Defensive, Château-Thierry, Saint-Mihiel and the Meuse–Argonne offensive. While in France, he was promoted to the rank of lieutenant colonel and then to colonel.