Jack was the leader of the nomadic biker gang that lead the raid on the Monroeville mall towards the end of Dawn of The Dead.
After he and Blades, the second in command of the gang, spot Stephen and Francine landing he helicopter on the roof of the mall, they decide to loot it later that night. After attempting to communicate with the survivors by radio, falsely claiming that there were only three of them, he and his gang launch an all out assault on the mall.
While Blades and the rest of the gang storm the mall, raiding supplies and killing the undead, Jack and the rest of his group wait outside, killing zombies to buy the bikers time.
After shooting at some undead, Jack is not seen again. Considering we do not see him devoured by zombies, nor shot by Peter during his skirmish, we can assume he retreated back to his van, and left with the remainder of his gang.