|April 20||Organized at Philadelphia and mustered in for three months service under Colonel John F. Ballier, Lieutenant Colonel Charles Ernenwein and Major Adolph Mehler|
|May 28||Moved to Chambersburg and attached to Geo. H. Thomas’ Brigade, 1st Division, Patterson’s Army|
|June 6||March to Greencastle|
|June 15||Cross Potomac and advance on Martinsburg Road|
|June 16-24||At Williamsport|
|June 25||At Downsville|
|July 2||Falling Waters|
|July 3||Occupation of Martinsburg|
|July 15||Advance on Bunker Hill|
|July 17||Moved to Charlestown, then to Harper’s Ferry|
|August||Moved by rail to Baltimore to Philadelphia|
|August 9||Mustered out.
Colonel Ballier was authorized to recruit another regiment for three years service, which become the 98th Pennsylvania Infantry. Many men of the 21st reenlisted in the 98th.