Eglin joined the Indiana National Guard in 1911 while a student at Wabash College, and first began federal service in June 1916, serving on the U.S. border with Mexico. In 1917 he was commissioned in the Indiana Guard after the United States entered World War I and was assigned to pilot training. After earning his rating as a pilot and a commission in the Aviation Section, U.S. Signal Corps, Eglin remained at the school as a flying instructor. Eglin received a regular commission in the Air Service on July 1, 1920 and commanded several squadrons in the United States and the Philippines.