Known for its colorful uniforms based upon the popular French Zouave style, the regiment served in the Eastern Theater.

After some garrison duty in their native Pennsylvania, the 23rd was assigned to the 1st Brigade, 1st Division of General Robert Patterson's Army of the Shenandoah.

By August, fifteen new companies had been raised, twelve from Philadelphia and one each from Pittsburgh, Pennsylvania, Wilkes-Barre, and Columbia.

The unit had not been assigned an active role in the fighting, but when it saw another regiment rout, it charged in without orders, helping carry Marye's Heights.

In the 1864 spring reorganization of the Army of the Potomac, the 23rd Pennsylvania was reassigned to the 4th Brigade, 1st Division, VI Corps.

At the beginning of the Overland Campaign, it guarded prisoners, rejoining the Army near the end of the Battle of North Anna.

After several days of additional skirmishing, the regiment was transferred with the rest of the corps to Bermuda Hundred and from there crossed the Appomattox River.

On July 9, the 23rd was transferred with the rest of the VI Corps to Washington, D.C., in response to Jubal Early's attack on the city.