The regiment was accepted for state and federal service as the "113th Volunteers" and its designation changed to the 12th Pennsylvania Cavalry on November 13, 1861.
It mustered in for three years service under the command of Colonel William Frishmuth.
The regiment was attached to Military District of Washington, to September 1862.
4th Brigade, Pleasanton's Cavalry Division, Army of the Potomac, to October 1862.
Averill's Cavalry Command, VIII Corps, Middle Department, to November 1862.
Defenses Upper Potomac, VIII Corps, to February 1863.
3rd Brigade, 1st Division, Department of West Virginia, to February 1864.
1st Brigade, 1st Cavalry Division, West Virginia, to August 1864.
Reserve Division, Department of West Virginia, to January 1865.
Moved to Manassas Junction, Va., and guard Orange & Alexandria Railroad until August.
Moved to Bristoe, then to Alexandria, and picket north bank of the Potomac River from Chain Bridge to Edward's Ferry until September.
Assigned to duty on line of the Baltimore & Ohio Railroad, Headquarters at Sir John!s Run and Bath.
Charlestown and Duffield Station June 29, Bolivar Heights July 2.
Guard and garrison duty at Charlestown, covering railroad from Harpers Ferry to Winchester until March 1865.