The 99th Regiment Pennsylvania Volunteer Infantry was an infantry regiment that served in the Union Army during the American Civil War.
Three companies moved to Washington, DC on August 8, 1861, and the regiment was eventually mustered in for a three-year enlistment under the command of Colonel Thomas Sweeney.
The regimental designation was officially changed in February 1862.
The regiment was attached to Defenses of Washington, D.C., to October 1861.
Jameson's Brigade, Heintzelman's Division, Army of the Potomac, to February 1862.
3rd Brigade, 3rd Division, III Corps, Army of the Potomac, to August 1863.
Moved to Harrison's Landing June 29-July 4, and duty there until August 16.
Pope's Campaign in northern Virginia August 26-September 2.
Burnside's 2nd Campaign, "Mud March," January 20–24, 1863.
—99th Pennsylvania's battle cry, witnessed by Corporal Peter Ears, Co. E, during the 99th Pennsylvania's bayonet charge at Devil's Den, Gettysburg, July 2, 1863[1] Pursuit of Lee July 5–24.
Demonstration on north side of the James at Deep Bottom July 27–29.
Mine Explosion, Petersburg, July 30 (reserve).
Demonstration on north side of the James River at Deep Bottom August 13–18.
Boydton Plank Road, Hatcher's Run, October 27–28.
Dabney's Mills, Hatcher's Run, February 5–7, 1865.