By June 17, 1918, McCormick became a lieutenant colonel, and by September 5, 1918, was promoted to a full colonel in the field artillery. He took part in the capture of Cantigny (hence his later naming his farm estate near Wheaton, Illinois), and in the battles of Soissons, Saint-Mihiel, and the second phase of the Argonne. McCormick served in the 1st Battalion, 5th Field Artillery Regiment, with the 1st Infantry Division. His service ended on December 31, 1918, though he remained a part of the Officers Reserve Corps from October 8, 1919, to September 30, 1929. Cited for prompt action in battle, he received the Distinguished Service Medal, and was later always referred to as "Colonel McCormick."