Originally, the regiment was to be raised as the United States Constitution Guard by Colonel John S. Cocks of the 2nd New York but organization was not completed.

At the Battle of Fair Oaks (Seven Pines) the 40th New York gained considerable recognition for its action on June 1.

The regiment's commander, Colonel Edward J. Riley, was kicked in the head and thrown by his horse prior to the battle, removing him from the front.

The Mozart Regiment suffered 96 casualties, including every member of the color guard killed or wounded.

[1] The 40th New York played a critical role in the defense of the Federal left flank during the second day at Gettysburg, delaying the approach of Law's Division to Little Round Top.