Sneden left Brooklyn in 1861 to enlist in the 40th New York Volunteer Infantry Regiment, or "the Mozart Regiment", of the Army of the Potomac. He served as a quartermaster when his regiment camped near Leesburg Turnpike. Starting from January 12, 1862, Sneden served on Samuel P. Heintzelman's III Corps staff, at first, as a draughtsman on map work, later, as a topographical engineer. On March 22, 1862, Sneden embarked with Heintzelman for the Peninsula Campaign, participating in the Battle of Williamsburg, Battle of Seven Pines, Battle of Savage's Station, and Battle of Glendale. Returning to Northern Virginia, he took part in the Second Battle of Bull Run. He was assigned to the defenses of Washington, D.C., first in Alexandria, Virginia, then at Arlington House.