When the Civil War began, White enlisted in the 12th Ohio Infantry, being chosen its lieutenant colonel. The regiment saw action its first at the battle of Carnifex Ferry in western Virginia. The colonel, John W. Lowe, was killed, and on June 28, 1861 White was made its colonel. The 12th Ohio was attached to Jacob D. Cox's Kanawha Division at the Second Battle of Bull Run and during the Maryland Campaign. The regiment saw heavy fighting at Fox's Gap and fought again in the vicinity of Burnside's Bridge during the Battle of Antietam.