After the Peninsula Campaign, the Pennsylvania Reserves joined the III Corps of the Army of Virginia, later I Corps in the Army of the Potomac. Seymour performed well at the battles of Second Bull Run, South Mountain, and Antietam in the latter half of 1862, especially in his brigade's capture of Frosttown Gap, Maryland, on September 14. Seymour took command of the Reserves after Brig. Gen. George G. Meade became acting corps commander. He received the brevet from the regular army of lieutenant colonel after South Mountain and that of colonel after Antietam.