Emigrating to Philadelphia with his parents while a boy, his youthful tastes inclined him to military affairs and he became active in the ranks of the militia.

In this campaign he was given the command of the picket line by Maj. Gen. Winfield Scott Hancock and covered the retreat of the Army of the Potomac across the Rappahannock River.

On May 4, 1866, President Andrew Johnson nominated Mulholland for the brevet grade of brigadier general of volunteers to rank from March 13, 1865, for his conduct at the Battle of the Wilderness and the U.S. Senate confirmed the appointment on May 18, 1866.

[6] Returning to civil life after the war, he was appointed Chief of Police in Philadelphia in 1868, and signalized his administration by the good order in which he kept both the force and the city.

He was considered an authority on the science of penology, and also devoted much of his leisure time to art studies, and as a lecturer and writer on the Civil War and its records.

Citation: In command of the picket line held the enemy in check all night to cover the retreat of the Army.Colonel Mulholland was portrayed by actor Timothy O'Hare[7] in the 2003 Civil War film Gods and Generals, in the scene depicting the charge of the Irish Brigade at Marye's Heights.