The 12th Pennsylvania Reserves was organized at Camp Curtin in Harrisburg, Pennsylvania beginning August 1861 and mustered in August 10, 1861 under the command of Colonel John H. Taggart.
The regiment was attached to 3rd Brigade, McCall's Pennsylvania Reserves Division, Army of the Potomac, to March 1862.
3rd Brigade, 2nd Division, I Corps, Army of the Potomac, to April 1862.
3rd Brigade, McCall's Division, Department of the Rappahannock, to June 1862.
3rd Brigade, 3rd Division, V Corps, Army of the Potomac, to August 1862.
3rd Brigade, 3rd Division, III Corps, Army of Virginia, to September 1862.
3rd Brigade, 3rd Division, I Corps, Army of the Potomac, to February 1863.
3rd Brigade, Pennsylvania Reserves Division, XXII Corps, Department of Washington, to June 1863.
3rd Brigade, 3rd Division, V Corps, Army of the Potomac, to June 1864.
Duty at Tennallytown, Md., August 13 to October 10, 1861, and at Camp Pierpont, near Langley, Va., to March 1862.
Ordered to Washington, D.C., February 6, and duty there and at Alexandria until June 25.