At the outset of the American Civil War, Montgomery organized and briefly was
colonel of the 1st Regiment New Jersey Volunteer Infantry. The regiment was held in reserve at the First Battle of Bull Run. Montgomery's biographical sketch in ''Appletons' Cyclopaedia of American Biography (1888)'' states that Montgomery's regiment "aided in covering [the Union Army's] retreat from Bull Run." Sifakis, on the other hand, states that the regiment was detailed to guard areas around Arlington Heights, Virginia and Fairfax Court House, Virginia. In June 1861, near
Trenton, New Jersey, Montgomery suffered rib injuries when his horse fell. Soon after the Battle of First Bull Run, Montgomery was appointed a
brigadier general of volunteers to rank from May 17, 1861. Thereafter, he served in administrative positions or was awaiting orders. Montgomery was military governor of Alexandria, Virginia, September–December 1861, in command at Annapolis, Maryland, January 1862 to March 17, 1862, in command at Philadelphia, Pennsylvania, April 4, 1862, to March 2, 1863; awaiting orders at Cairo, Illinois, March to October 1863 and on a military commission at Memphis, Tennessee from October 16, 1863, to March 15, 1864. Montgomery was on leave of absence from March 15 to April 4, 1864. He submitted his resignation from the Union Army, presumably due to ill health according to Warner, which was accepted on April 4. 1864. == Later life ==