In April 1914, McNair was promoted to permanent captain, and from April to November 1914, McNair took part in the Veracruz Expedition as the 4th Field Artillery Regiment's commissary officer. Assigned following another by name request, this time from regimental commander Lucien Grant Berry, McNair was responsible for procuring, storing, maintaining, accounting for, and distributing the regiment's materiel, including equipment and weapons. During 1915 and 1916, he was again assigned to the Field Artillery School, where he continued to work on procedures for the implementation of the firing table data he had published. He also continued to experiment with different types of artillery pieces so he could make manufacturing and procurement recommendations as the Army began to prepare for possible involvement in World War I. He returned to the 4th Field Artillery Regiment for the Pancho Villa Expedition on the Texas-Mexico border; initially an unassigned staff officer available for additional duties, he later commanded a battery. In May 1917, he was promoted to major and assigned to temporary duty as an instructor for the officer training camp at Leon Springs, Texas.