During his youth and like his father before him, Cormac was trained by his grandfather, learning to master Freerunning and Eagle Vision. Later, he also inherited the rifle of the Cormacs, gifted by Benjamin Franklin to his ancestor.
By 1863, Cormac was the feared barman of one of the city's pubs, where he was acting as the Templar contact with the gangs, being a trusted agent of Grand Master William M. Tweed. As such, he was tasked to convince the other gangs to participate in the riots organized by the Templars to turn the tide of the American Civil War in their favor. The night before the riots were scheduled to start, a letter of Tweed was delivered to Cormac, instructing him to stop an Assassin from stealing a Piece of Eden rumored to be hidden in the Aztec Club and to bring the artifact to his house.
Later, the Templar ambushed the Assassin Varius as he was leaving the Club with a Dagger of Eden, taking the artifact from his paralized foe but arrogantly decided to spare him. He then left as the riots were starting in the city, determined to bring the artifact to his master after sunset and prudently stayed out of the troubles, witnessing the riots from a church's bell tower. While agreeing that the Black citizens of New York were a necessary, yet sad, casuality of the Templar agenda, Cormac nevertheless decided to save the children of a black orphanage after it was set ablaze by rioters.
Eventually, as he reached Tweed's estate, the Templar discovered that guided by one of Tweed's servant, Eliza, Varius was also in the house and the two resumed their fight. They Freerunned on the roofs of the city while brawling, until Cormac inadvertently used the power of the artifact on Varius while ranting about his Templar ideals, throwing a knife at the confused Assassin, which hit him in the stomach.
However, as Cormac was about to finish the critically injured Assassin, Eliza, who followed them, used the rifle that Cormac lost during the fight to shoot a paralizing dart at the Templar. Enraged by the death of her father, beaten to death during the riots, Eliza threatened the powerless Templar which she knew was responsible, putting the Dagger on Cormac's throat. She nevertheless spared him, promising to return for him and Tweed later, before vowing to join the Assassins and to accomplish the mission of Varius, delivering the Dagger to General Ulysses S. Grant.