Simpson was promoted to captain on May 15, 1917, a month after the American entry into World War I. He followed Bell on a tour of inspection of the British and French forces on the Western Front in September and October 1917. They then returned to Camp Logan, Texas, where the 33rd Division was activated. The 33rd Division embarked for France on May 16, 1918. Simpson became its Assistant Chief of Staff (G-3), the staff member responsible for operations. He was promoted to major on June 7, 1918, and attended the American Expeditionary Force Army General Staff College from June 15 to August 30. He returned to the 33rd Division as assistant to its G-2 (the staff member responsible for intelligence) on September 1, then became assistant to its G-3 on September 15. He became G-3 again on October 4, and participated in the Meuse-Argonne Offensive. He was promoted to lieutenant colonel on November 5, and became the division chief of staff on November 17, soon after World War I ended on November 11. He was awarded the Army Distinguished Service Medal and Silver Citation Star, and the French Croix de guerre and Legion of Honour in the grade of Chevalier.