Under McClellan, Ewing commanded a regiment and then a brigade in the Kanawha Division in the IX Corps. In the Battle of South Mountain, he led the assault which drove the enemy from the summit; and at midnight of that day, he received an order placing him in command of the brigade of Colonel Eliakim P. Scammon, who was in temporary command of the Kanawha division after its commander Major General Jacob D. Cox had been elevated to command of the IX Corps, replacing the fallen Major General Jesse Lee Reno who was killed earlier that day. At Antietam his brigade was placed upon the extreme left of the army, where, according to the report of the commander of the left wing, General Ambrose Burnside, "by a brilliant change of front he saved the left from being completely driven in."