After arriving in Washington, the regiment became part of Major General Irwin McDowell's Army of Northeastern Virginia and saw their first combat during the Battle of Blackburn's Ford. The 1st Massachusetts was engaged during the First Battle of Bull Run. When Union forces surrounding Washington were reorganized, the regiment became part of the Army of the Potomac, with which it was associated for the rest of its term of service. It was involved in the Peninsular Campaign and was present for virtually all of the major battles in which the Army of the Potomac fought, including the Second Battle of Bull Run, the Battle of Fredericksburg, the Battle of Chancellorsville, the Battle of Gettysburg and Lieutenant General Ulysses Grant's Overland Campaign.