Occupation in civilian life was that of a laborer.
Died in the city of San Fransisco.
Enlisted on 24 September 1871 in the 7th U.S. Cavalry.
At the time of the Battle of the Little Big Horn he was a Sgt. in James Calhoun (Custer's brother-in-law) L troop.
Each company had selected a few men to remain behind with the pack trains to assist their movement. That is no doubt what saved Mullen's life that day. L troop was wiped out.
Mullen came up with the pack train to what is now known as Reno Hill. He would participate in the fighting there with Reno, Benteen and Weir and their troops until the battle was over.