The regiment was organized at Camp Curtin in Harrisburg, Pennsylvania, in the summer of 1861, with John R. Brooke, of Pottstown, Montgomery County, was commissioned colonel on August 21.

On March 21, Brooke's command supported General Oliver O. Howard's Philadelphia Brigade in its reconnaissance to Warrenton Junction and beyond to the Rappahannock River.

While here, the 64th New York was temporarily attached to the regiment for drill, discipline, and camp duty, all under command of Major Octavius S. Bull, promoted to fill the vacancy left by the death of Yeager.

On October 30, the division crossed the Shenandoah River and proceeded down the Loudoun Valley, skirmishing with Confederate troops at Snicker's Gap on November 4.

Despite suffering heavy casualties and running low on ammunition, Brooke held his position, withdrawing back into the city after nightfall.

On April 28, the Union Army began the Chancellorsville Campaign, crossing the Rappahannock River at United States Ford and moving into the Wilderness.

The regiment was actively engaged on the skirmish line during the Battle of Chancellorsville, at one point in support of a battery near the Chancellor House with the 2nd Delaware and 145th Pennsylvania.

Having found that the Confederate columns had passed, the Fifty-third moved with the army northward, including a forced march to Thoroughfare Gap on June 20.

Luckily the corps had passed through the gap hours earlier and the regiment soon rejoined its division as they crossed the Potomac River and marched to the vicinity of Frederick, Maryland.

The division marched down a narrow farm lane to the north side of the field, where it was formed into battle lines and the brigades sent in one by one to stop the Confederates.

On the 14th it was deployed in line at right angles to the Williamsport Road and advanced cautiously only to discover the rebel works vacant, the southerners having crossed the Potomac River the evening before, back into Virginia.

The Fifty-third was detached from the brigade to guard the corps' wagon train until that afternoon when it marched to rejoin the division, in line behind earthworks constructed along the length of the Brock Road.

The Fifty-third moved to the left approximately two miles and threw up breastworks before being recalled to the former position, where they were shifted again to the right to relieve the 19th Maine Infantry on the firing line.

A massive Confederate charge by General James Longstreet's Corps was halted at the Union works, a portion of which went up in flames, but the line held.

(Dimm mustered out of service due to wounds received in September 1864 and was replaced by Captain William M. Mintzer [fr], promoted to major.)

Crossing on the 24th, the corps seized a ridge overlooking the river and southern lines, which was held until five o'clock when the Fifty-third moved forward three-quarters of a mile and built breastworks.

The regiment participated in the massive assault during the Battle of Cold Harbor the next day, suffering severe losses due to artillery and musket fire.

The regiment formed line of battle on the left of the brigade, now commanded by Colonel Beaver of the 148th Pennsylvania, and moved forward with bayonets fixed.

Apart from a foray to Reams Station in mid-July, the regiment was busy with the building of entrenchments until July 26, when the Fifty-third moved with the brigade to the right and north of the James River, to take part in the First Battle of Deep Bottom.

On the 21st, the regiment re-crossed the James and the Appomattox Rivers and passing in rear of the army to the extreme left of the line, commenced demolishing the Weldon Railroad near Ream's Station.

Five miles had already been destroyed when the Confederates attacked in force and a line of battle was hastily formed to repel his advance and protect the working parties.

Exhaustion, illness and low morale all contributed to the near disaster for the Second Corps at the Second Battle of Ream's Station, forced to abandon the field and retire to the lines in front of Petersburg.

On March 29, 1865, the V Corps had been ordered to move westward around the strained southern lines toward Five Forks, a major crossroads that protected the vital South Side Railroad into Petersburg.

The advance continued the next morning, driving in Confederate skirmishers across Dabney's Mill Roadto the Boydton Plank Road before dark, where the men threw up breastworks and camped for the night.

As the Battle of Dinwiddie Court House progressed, Miles ordered the Fourth Brigade to form a line in the rear to halt the retreat of the V Corps, which the men did as directed.

On April 2, the Fifty-third occupied abandoned Confederate works on the White Oak Road, and then marched in pursuit of the retreating force, confronting a dense line near Sutherland's Station.

Six hundred prisoners, one battleflag, and two artillery pieces were captured by the division in the Battle of Sutherland's Station, at a cost of eight men wounded in the Fifty-third.

The regiment moved out the following morning, crossed the South Side Railroad near High Bridge and over the Appomattox River where they immediately formed line of battle to confront a Confederate rear guard.

One man was killed and another wounded in the ensuing Battle of Cumberland Church, which netted over one hundred prisoners, along with supplies and equipment scattered on the Buckingham Road toward Farmville where the Fifty-third bivouacked for the night.

For the success attained in this service, Colonel Mintzer was promoted brevet brigadier general and several other officers received brevet-promotions.