Little is known about Jack's past, other than he was a hitman with the McCrellan Corporation, a predominantly Irish mob in Boston, but quit to testify against them. He and his daughter Lindsey were in the witness protection program for ten years, under the protection of U.S. Marshal Pat Mannan. His wife was later killed in a car crash intended to kill him. Considering that he was in witness protection and his entire background was taken out of a book, it is extremely likely that Jack Vaughn isn't even his real name.
Lindsey and her friend, Katie Owen, were approached at a movie theater by three males, one of whom Katie had a crush on. While Katie wanted to go with them, Lindsey didn't trust them but went anyway because she was protective of Katie, who would later be tortured and strangled to death in an abandoned house by the leader while Lindsey was forced to watch. The BAU is called in to assist in the investigation and are unaware that Jack is a hitman. They later discover that Mannan is a U.S. Marshal when Morgan sees a gun on him. This is while he is struggling to restrain Jack from attacking Rossi, who implied Lindsey had been abused in the past, just after Reid discovered that the personal history they have been sharing with their friends and acquaintances is a direct extrapolation of the plot of a novel, including the names they are using. At the police station, Bruce, Katie's father, knows the dead suspect, as well as who his friends are, and asks Jack if he is still up to doing what he "used to do".
Jack then overpowers Mannan, disappears, and ambushes with one of the abductors, shooting him in the leg with a Remington 870 shotgun stolen from the latter's home. After questioning the injured young man, Jack is told the location of Lindsey and her other abductor, Ryan Phillips. Meanwhile, Reid ascertains the location himself and rushes to meet the rest of the team there. When he arrives, he sees Jack holding Ryan at gunpoint as his daughter looks on. Reid tries to get Jack to drop the shotgun, but Lindsey begs her father to kill Ryan. Jack agrees that he needs to make changes in his life to make a better life for his daughter. Reid asks, "When is the violence going to end?" Jack replies, "Tomorrow." He pulls the trigger, killing Ryan. The rest of the team arrives with Mannan, and they reason that what happens to Jack depends on how important of a witness he is. Jack is later seen living a new life with his daughter in Atlanta, Georgia. They meet a pair of neighbors, a father and his daughter. Both the men and the girls shake hands and Lindsey introduces herself as "Katie".
Sometime after the move to Atlanta, Jack and Lindsey were relocated to Beloit, Wisconsin, after the McCrellan Corporation was apparently dismantled for good. Though Jack moved on with his life, Lindsey left witness protection five years prior to the episode and followed in his old footsteps, stealing his old service pistol and became a hitwoman for the drug cartels. Ten years later, Jack is approached at a park by Alvez and Stephen. They ask him if he has seen Lindsey, but he replies that he has not. They inform him that Lindsey had become a hitwoman and that she is now trying to frame Reid for murder. Then, Stephen tells Jack that they will get Lindsey and she will be locked up for life, but he can still see her. Again, Alvez asks Jack if she contacted him recently. He tells him that he talked to Lindsey two months ago, and that she wanted to know about the dissociative agents scopolamine and sevoflurane.
No specific details were given about Jack's M.O. during his time as a hitman in 3rd Life. It was later revealed in Season Twelve that his weapon of choice during that time was a .50-caliber IMI Desert Eagle handgun.
When he went after the men who abducted Lindsey, Jack surprised Taylor Coleman in his home and stole his father's Remington 870 shotgun before shooting him in the leg. He threatened to kill Taylor if he didn't tell him where Lindsey was being held but spared him when Taylor told him where to find her. Arriving at the school, Jack ambushed Ryan Phillips by bludgeoning him in the face with the same shotgun. Moments later, after some hesitation, Jack ultimately decided to kill Ryan by shooting him in front of Reid and Lindsey, under the latter's orders.
No official profile of him was made by the BAU, since he was a witness and not an unsub.
- Killed numerous unnamed victims during his time as a hitman
- January 6: David Rossi (attempted to assault)
- January 7:
- U.S. Marshal Pat Mannan (assaulted and stole his car)
- Taylor Coleman (one of his daughter's abductor's; attempted, but survived; was non-fatally shot in the leg)
- Ryan Phillips (another one of his daughter's abductor's; bludgeoned with a shotgun, then fatally shot with the same shotgun on Lindsey's orders)
- Jack currently holds the record for the longest gap between appearances for a recurring character, with a total of 219 episodes between 3rd Life and Green Light. It was previously held by his daughter Lindsey Vaughn, who appeared 218 episodes after her debut in 3rd Life before returning in Unforgettable.