The regiment was led by Colonel Charles Prevost until he was seriously injured at the Battle of Shepherdstown in which Lieutenant-Colonel James Gwyn assumed command until the end of the war.

The regiment initially rendezvoused at Camp Union at Philadelphia, where it was mustered into Federal service on August 30, 1862, for a three-year term.

[3] The regiment moved south in early November and took part in the Battle of Fredericksburg, where it joined in the assault on Marye's Heights and suffered severely.

During the infamous "Mud March" in January, 1863, the 118th was involved in a riot, fueled by a whiskey ration, with two other units, the 22nd Massachusetts and the 2nd Maine.

[citation needed] The men of the 118th PA and the 22nd MA came to blows after accusations flew that the Massachusetts regiment failed to support the Pennsylvanians at Shepherdstown.

[citation needed] The 118th was in camp at Falmouth until June 10 when the Army of the Potomac commenced its northward movement for the Gettysburg Campaign.

At four o'clock that afternoon, the 118th went into action in support of Maj. Gen. Daniel Sickles' III Corps and was closely engaged, losing 3 killed, 19 wounded, and 3 missing or captured.

On April 1, it participated in the Battle of Five Forks and continued the pursuit to Appomattox Court House, where its brigade received the arms and flags of General Robert E. Lee's army.

[2] A group based in the United Kingdom currently represents Company C and belongs to the American Civil War Society.