Formed mostly from a militia unit in Norristown in southeastern Pennsylvania, the regiment enlisted at the beginning of the American Civil War in April 1861 for a three-month period of service under the command of Colonel John F. Hartranft.

Logistical difficulties bedeviled the regiment, which served as part of the garrison of Washington, D.C., until late June, when it was sent into Northern Virginia to join in the army of Brigadier General Irvin McDowell.

The regiment suffered its only combat casualties in a picket action on June 30 and was sent back to be mustered out on the eve of the First Battle of Bull Run owing to disagreement among the men over remaining with the army after the expiration of their term of service.

President Abraham Lincoln called for 75,000 men to serve in the army for three months after Confederate forces began the American Civil War by firing on Fort Sumter, and in response a mass meeting was held at the Odd Fellows Hall in Norristown on April 16.

After being presented with flags sewn by women of the town and given a send-off from the population, the Norristown companies moved to Harrisburg by rail and entered Camp Curtin the same day.

The blouses and pants that they received, provided to the state by war-profiteering contractors, were "made of damaged goods of inferior quality," as observed by industrialist Benjamin Haywood, dispatched by Curtin to investigate after widespread complaints.

[2] The 4th Pennsylvania was sent to Alexandria, Virginia, where it was encamped on Shuter's Hill at a site named Camp Hale, in readiness for a Confederate attack, on June 20.

The regiment became part of the Colonel William B. Franklin's 1st Brigade of Samuel P. Heintzelman's 3rd Division of the Army of Northeast Virginia, which was commanded by Brigadier General Irvin McDowell.

In the preliminary movements of the Bull Run campaign, the division left camp on the Old Fairfax Road, arriving at Sangster's Station late on July 18.

Their march was so disorderly that I could not have believed they were soldiers in an enemy's country...but for their arms and uniform...they were all in good spirits, but with an air about them I could not understand...I asked an officer "Where are your men going, sir?"

The appeals of McDowell and Hartranft to patriotic duty were partially successful: many in the regiment were willing to stay, but others wanted to muster out as scheduled due to their previous negative experiences with lack of equipment.

[10] Hartranft and Captain Walter H. Cooke of Company K stayed with the army, serving on the staffs of Franklin's brigade and David Hunter's division, respectively.

[2] Cooke, after finding that only a half dozen of his men stepped forward to fight in response to his question, left in disgust and initially started for the camp of the New York Fire Zouaves to serve as a private before being told he could be more useful with the staff of a unit.

The 4th Pennsylvania was not in unanimous agreement on departing, Corporal Joseph K. Corson of Company K later recounting that he was ashamed of marching away from the sound of the guns, and that others felt similarly.

[20] Another officer who continued his service with the 51st was the first lieutenant of Company H, William H. Blair, who was brevetted brigadier general for his actions in the storming of Burnside's Bridge at Antietam.

[22] Company A Private George Morton Randall joined the Regular Army in the fall of 1861 and rose to major general after the war.