Clements' studies were interrupted by World War I. On July 9, 1917, he enlisted as a private in Company M of the Kentucky National Guard. The company was ordered to Camp Taylor near Louisville, Kentucky where they were mustered into the infantry of the U.S. Army. Clements first served as a guard at Camp Taylor and later entered the Officers Training School at Fort Benjamin Harrison near Indianapolis, Indiana. He graduated with the rank of first lieutenant and remained stateside as a professor of military science. He served for a total of 28 months, attaining the rank of captain, and was discharged on September 12, 1919.