He later commanded the brigade, including at the Battle of Gettysburg where Brewster and his men were overrun by Confederates while defending the Emmitsburg Road position of the III Corps not far from the Peach Orchard.

When the war broke out, Brewster was commissioned as the major of the 28th New York State Militia at Brooklyn in the spring of 1861.

Brewster returned to New York, where he became involved in the recruiting efforts of the controversial politician Daniel Sickles, which led to creation of the Excelsior Brigade.

When he returned to the Army of the Potomac, Brewster found himself in command of the Excelsior Brigade in the division of Brig.

The remains of the brigade took part in a counterattack late on July 2 that recaptured some abandoned Union guns.

[2] When the Army of the Potomac was reorganized for the Overland Campaign of Lt. Gen. Ulysses S. Grant, Brewster retained brigade command.

The brigade, augmented with two regiments, served in the Battle of the Wilderness in the Fourth Division, II Corps under Brig.

Gen. Régis de Trobriand joined II Corps during the siege, he took command of the First Brigade, with Brewster as the senior colonel.

When de Trobriand was acting division commander October 8-October 21, 1864, Brewster led the brigade in his place.