The original Company B, also known as the Union Guards, was comprised of volunteers from Lancaster County in south central Pennsylvania. The company--with a majority of its members also being members of the Union Fire Company in Lancaster--derived their name from that fire company which was located on East King Street. As part of the Pennsylvania Reserve VolunteerCorps, they belonged to a division which was unique in that all were from the state of Pennsylvania. At various times during their enlistment the Reserves were commanded by such famous generals as George Gordon Meade, Samuel Wiley Crawford and Lancaster's own John F. Reynolds. Gaining a solid reputation as a fighting unit, the 1st Reserves fought at most of the major engagements in the eastern United States during their three years of service. They fought as members of the Army of the Potomac in both the First and Fifth Corps.

The unit first distinguished themselves in the Peninsular Campaign in 1862. At Second Bull Run the Reserves fought gallantly, despite the Union Army's setback. Engaged heavily at South Mountain and in the Cornfield at Antietam, the regiment suffered severe losses. At Fredericksburg they were part of the only division to penetrate enemy lines.