August 16-24 : The 138th is formed at Camp Curtin in Harrisburg Pennsylvania. The first troops arrive at the camp starting on the 16th and the entire unit is complete by the 26th. The troops that make up the unit are from Adams,Bedford,Bucks,and Montgomery counties.
August 30: The unit is stationed at the Relay House in Baltimore Maryland and will remain here until spring.
March 30: Sunwalt's command of the unit is terminated for insubordination.
May 2:Lieutenant Colonel M'Clennan was promoted to full colonel and assumes command
June 6: The unit is moved to Harper's Ferry Virgina and becomes an active unit under General French's 3rd Division 2nd Corps.
July 1: Harper's Ferry is evaccuated and the 138th guards the ammunition as it moves to D.C.
July 16: The 138th will cross the river and pursue Lee's retreating Army of Northern Virginia.
July 23: Skirmish at Wapping Heights
November 8: Battle of Brandy Station. This is the 138th's first real engagment.
November 23-27: Mine Run Campaign. After these engagments the regiment will set up winter quarters at Brandy Station.
February 6-7: Demonstration on the Rapidan
May 5-7: Battle of the Wilderness
May 7: Unit marches to Spotsylvania where it will encounter very little action.
May 12: Assault on the Salient
May 28-31: Totopotomoy
May 31-June 3: Battle of Cold Harbor
July 9: Battle of Monocacy. Regiment is made part of Sheridan's Middle Military Division after the battle
July 14-24: Pursuit of Jubel Early to Snicker's Gap
September 19: Battle of Opequan
October 19: Battle of Cedar Creek
April 2: Final assault on Petersburg
April 6: Battle of Sailor's Creek-The 138th's last battle
April 9: Lee Surrenders
April 23-27: March to Danville
May 10: Corporal Trustrim is cited for the Medal of Honor
June 23: The 138th PA Volunteer Infantry Regiment is mustered out
June 27: The members of the unit are transported to Harrisburg and recieve their final pay and are discharged