Mainly composed of infantry regiments raised in the state of New York primarily by former U.S. Representative Daniel Sickles, the brigade served in several of the Army of the Potomac's most important battles in the Eastern Theater, including Chancellorsville and Gettysburg.

At the outbreak of the Civil War, the controversial Sickles desired to repair his public image, which had been marred by the shooting death of his wife's paramour, Philip Barton Key.

Sickles was active in raising thousands of recruits from around New York City for service in the United States volunteers in May 1861 under the authority of the War Department.

On the morning of June 25 at the Battle of Oak Grove, Sickles's New Yorkers encountered difficulties moving through their abatis, then through the upper portions of an impeding swamp, and finally met stiff Confederate resistance, all of which threw the Federal line out of alignment.

[5] Sickles was again absent for the Second Battle of Bull Run, having used his political influences to obtain leave to go to New York City to recruit new troops.

The Excelsior Brigade, still under Colonel Taylor as Sickles had been promoted to divisional command, missed the Battle of Antietam in September because the III Corps was stationed on the lower Potomac River, protecting the capital.

With the rest of Hooker's old division, it was held in reserve in some woods near the Chancellor House, guarding a road that led to the important United States ford over the Rappahannock River.

Spinola's brigade led the Union troops on July 23 at the Battle of Wapping Heights in Linden, Virginia, suffering 18 men killed, including two officers.

The Excelsior Brigade, augmented with the 11th Massachusetts and the 84th Pennsylvania, served in the Battle of the Wilderness in the Fourth Division, II Corps under Brig.