The Ninth Regiment was created on 11 June 1861 under the command of Colonel Thomas Cass in Boston recruiting primarily Irish-Americans.

On 26 June 1862 the regiment participated in the Battle of Mechanicsville, about six miles from Richmond Virginia which resulted in a Union Army victory and more than 2,000 Confederate casualties.

The support troops were delayed in their arrival new and the Ninth Regiment successfully repelled several Confederate assaults on the bridge.

The Ninth participated in the Battle of Malvern Hill on 1 July 1862 as part of the Peninsular Campaign waged by the Union Army intending to capture the city of Richmond.

Supported by five US Navy gunboats on the nearby James River, and benefiting from favorable geography and extensive defensive preparations, the Union Army withstood repeated charges inflicting heavy casualties on the Southern forces.

The unit's casualties were very heavy; along with losing their two top commanders, roughly half the regiment was put out of action, totaling 166 men.

Colonel Cass died in Boston Massachusetts on 12 July 1862 and was buried with full military honors at Mt Auburn cemetery.

News of the defeat at Bull Run so shook the inhabitants of Washington, D.C. that plans were made to abandon the city before the arrival of the Confederate Army.

With the help of substantial stone breastworks, the regiment successfully withstood several assaults by the Confederate Army, taking casualties of 15 killed, wounded, or missing.

The regiment continued to fight at North Anna River and Bethesda Church near Cold Harbor with light losses.