The regiment was formed with 800 men in 1861 and was placed in a brigade along with the 24th New York, 30th New York, 84th New York (14th Brooklyn) (also known as the 14th New York State Militia), and for a number of months, the 2nd U.S. Sharpshooters (a regiment under the respected Colonel Henry A. V. Post). The brigade was the First Brigade in the First Corps of the Army of the Potomac, and Colonel Walter Phelps and the regiments of the brigade referred to themselves as the Iron Brigade of the East and served with the Division's more famous Fourth Brigade which would earn the title "Iron Brigade of the West" in September 1862 during Battle of South Mountain in the Maryland Campaign.