Their recruits were volunteers, the majority of whom initially enlisted for three-year terms of service from their hometown of Philadelphia; they were divided into 10 lettered companies upon muster in during early December 1861: A, B, C, D, E, F, G, H, I, and K. (In later years, as the war dragged on and the need for reinforcements arose, additional men were drafted from communities across the state.

[11] Six days later, the regiment was ordered to Alexandria, Virginia, where it relieved the 88th Pennsylvania and was assigned to provost (military police) duty with Col. Gregory and Capt.

[14] Relieved by the 94th New York on August 21, the 91st Pennsylvania was attached next to the U.S. Army's 5th Corps, 1st Brigade, 2nd Division which had been ordered to join Gen. John Pope's forces.

Stationed near Cloud's Mills, Virginia until August 28, the 91st Pennsylvania then served as an escort for eighty-seven wagons being moved to the Fairfax Court House, but was ordered back to camp upon reaching Annandale.

[15][16] Next engaged in duties on the Monocacy Creek as part the Union's Maryland Campaign beginning September 15, the 91st Pennsylvania moved to Antietam three days later.

[17][18] Connecting with the Third Division on October 16, the 91st crossed the Potomac River on a valley reconnaissance mission, during which it skirmished with Confederate States Army (CSA) troops at Shepherdstown.

On October 30, the regiment was moved to Warrenton as part of the Union Army's reorganization during the leadership transition from generals George B. McClellan to Ambrose Burnside.

Encamped at Stoneman's Switch from mid-November to December 11, the 91st Pennsylvania then marched to the Phillips House as part of the lead-up to the Battle of Fredericksburg.

Ordered to support the 2nd Corps, regimental leaders marched their men through Fredericksburg, and stationed them behind the stone walls of a cemetery outside of the city, a post they held until ordered onto the Union's Fredericksburg Road line, where they were subject to heavy artillery fire and sustained several casualties, including the combat deaths of Lt. George Murphy and Maj.

Four hours later, it was ordered back out to the front, where it assisted in transferring wounded men from where they had fallen to the care of Union medical personnel, working until sunrise.

[26][27] Beginning January 20, 1863, the 91st Pennsylvania joined Gen. Ambrose Burnside's Mud March during which it helped build corduroy roads to facilitate troop and equipment movements.

By this time, the regiment was under the command of Joseph H. Sinex, who had been placed in charge following the departure of Lt. Col. Wallace, who had been honorably discharged on a surgeon's certificate of disability on January 10.

[28][29] At the end of February, the Alexandria Gazette noted that the regiment had presented its commanding officer, Col. E. M. Gregory, "with an elegant sword, and a splendid horse, in token "of their approbation of his gallant conduct at the battle of Fredericksburg, Dec.

[33][34] Ordered back to Chancellor House, the 91st Pennsylvanians advanced into a tangled, wooded area, where they skirmished briefly with CSA troops before moving on.

Leaving dead and wounded men behind, they were assaulted by intense artillery fire during their retreat, but were able to regroup and be reposted at the fork of the Chancellorsville and United States Ford roads, where they remained until noon the next day (May 4) when they were ordered to move to the rear of the Union's lines and resume work on fortifications.

[35][36] Relieving the 32nd Massachusetts at Stoneman's Switch on May 28, the 91st Pennsylvanians guarded the railroad from the station to Potomac Bridge before being assigned to cavalry relief on June 4 at United States Ford.

Ordered to head for Frederick City by way of Manassas Junction, Gum Spring and Aldie, they crossed the Potomac at Edwards' Ferry, they then continued on to Hanover, Pennsylvania, arriving during the evening of July 1, 1863.

[39][40] At 2 p.m., they marched for Little Round Top to relieve a segment of the beleaguered 3rd Corps, and were soon engaging CSA sharpshooters positioned in the Devil's Den.

[52][53] Their furloughs over on February 16, the 91st Pennsylvanians reported back to headquarters, and were then moved to the Upland Institute in Chester, where they remained until leaving for winter quarters at Warrenton Junction on March 2.

According to Bates' History of Pennsylvania Volunteers, during this phase of duty, regimental leaders used this time to replenish their ranks, enrolling additional recruits from mid to late March (five of whom were categorized as "Unassigned Men").

[58][59] During the morning of May 12, the 91st Pennsylvania staged a diversion in the face of heavy rifle and artillery fire which enabled the 2nd Corps to effect a larger, surprise charge on the CSA.

Following a brief respite from the action, during which they were relieved by a 6th Corps brigade which ultimately lost the ground they had just gained, they returned with the 140th New York to recapture the area around the court house.

[65][66] Assigned to strengthen Union fortifications during most of July and to garrison detail from the end of that month until August 18, the 91st Pennsylvanians briefly engaged the CSA near the Weldon Railroad.

After building additional fortifications in the area in early October, they drove off CSA troops stationed at Davis House, and burned the structure.

[69][70] While attempting once again to destroy the Weldon Railroad in early January 1865 and engaging the CSA again at Hatcher's Run on February 6, the 91st Pennsylvania sustained further casualties, including Capt.

[71][72] The war technically over but the preservation of America's Union not yet completely assured, the regiment was subsequently marched to Petersburg and Sutherland Station, where it remained until May 4 when it was ordered to Richmond and then Bailey's Crossroads.

Erected on September 12, 1889, at the highest point of Little Round Top, "this monument appears to hang over the western edge of the hillside, just off the asphalt path that winds throughout the summit," according to park officials, and documents the exact position defended by the regiment from July 2–3, 1863.