Initially assigned to I Corps, in June 1862, the division was transferred to the Virginia Peninsula where it served with the V Corps of the Army of the Potomac during the Peninsula Campaign. The division then returned to its old corps (which was at the time designated III Corps in the Army of Virginia) during the Second Bull Run Campaign. The division's parent formation resumed its I Corps designation just prior to the Antietam campaign when it rejoined the Army of the Potomac. The division participated in the battles of South Mountain, Antietam, and Fredericksburg, particularly distinguishing itself at Fredericksburg, where it penetrated the Confederate lines. Between Fredericksburg and Gettysburg, the division was a part of the XXII Corps assigned to Washington, D.C. It was part of the V Corps again for the Battle of Gettysburg, where it distinguished itself on July 2, 1863, fighting around Little Round Top. One brigade drove Confederate forces from the western slopes of Little Round Top back to the Wheatfield. Under Crawford, the Pennsylvania Reserves continued to fight with the Army of the Potomac until just before the Battle of the Bethesda Church or Battle of Totopotomoy Creek, when the men's three-year enlistments expired.