At the beginning of the American Civil War, Montgomery organized and was briefly colonel of the 1st Regiment New Jersey Volunteer Infantry. The 1st New Jersey Volunteer Regiment was "held in reserve" at the First Battle of Bull Run on July 21, 1861, according to Wilson and Fiske. Sifakis states that Montgomery's regiment was detailed to guard areas around Arlington Heights, Virginia and Fairfax Court House, Virginia. Cullum also supports this view by stating that Montgomery first served in the defenses of Washington, D.C. at the start of the war.