 Manning's teaching career was interrupted by the U.S.-Mexican War, and he left for Texas in June, 1916 as commander of the Charleston Light Dragoon, a National Guard unit.Following his return from Texas, Manning served briefly as Commandant of Cadets at the Porter Military Academy (now the Porter-Gaud School), before leaving to serve as an artillery officer in World War I.

Manning served with the 316th Field Artillery in France and was promoted all the way to Lieutenant Colonel before returning to South Carolina in 1919.He worked as a farmer and businessman until beginning his political career in 1930.