|“||You know, you have no idea who or what you're dealing with here.||”|
— Marshal Mannan
Little is known about Mannan's personal life, other than he became a U.S. Marshal. At some point in 1998, he was assigned to protect former hitman Jack Vaughn and his family, as Jack worked with the McCrellan Corporation, a predominantly Irish mob in Boston, but quit to testify against them. Jack's wife was later killed in a car crash intended to kill him.
In January 2008, Jack's daughter Lindsey is abducted alongside her friend Katie Owen by a trio of males. Katie is later killed by the group's leader and her body dumped with her face and hands mangled. Following the discovery of the body, Morgan and JJ meet Mannan, Jack, and Katie's parents, who were blocked from the crime scene, and Mannan introduces himself as Jack's friend. Later, the BAU find an audio recording of Katie's murder and announce this to Mannan, Jack, and Mr. and Mrs. Owen. They then suggest appealing to the public for help; Mannan advises Jack to disregard that, claiming that these live news conferences are set up by the BAU so they could observe the parents' reactions and check for signs of guilt. Immediately afterward, the BAU have Jack, Mannan, and Katie's father listen to the recording, and they realize that it was Katie who was murdered.
Later, Reid, Morgan, and Rossi search Jack's house for any clues while Mannan and Jack watch. The three agents ask about Lindsey's emotional state following her mother's death; when Mannan and Jack deny that she was highly affected, they profile that she was a shy and reserved teenager, judging by the appearances of her bedroom. Their deduction skills impress Mannan. Soon after, they find Reid investigating the family computer and discovering that they were using a proxy server that would make tracking online histories difficult. The agents then confront them about the strangeness of the Vaughan home and the fact that Lindsey was exhibiting unconventional behavior in the audio recording. When Rossi suggests it was either because she was coached or experienced sexual abuse before, Jack snaps and tries to attack Rossi. Mannan stops him, but accidentally reveals his concealed handgun in the process. Held at gunpoint, he voluntarily surrenders his weapon and then reveals himself as a Marshal protecting the Vaughans.
When Hotch is informed of this, he has Jack and Mannan sent to the local police station for observation. He later watches as Jack explains his past to the BAU, and he also reveals to the team that the trial against the McCrellan Corporation is scheduled to begin in two weeks. Sometime later, they learn that the McCrellan Corporation had nothing to do with Lindsey's abduction, and that one of her abductors was found dead. Mannan attempts to comfort Jack, saying that this had nothing to do with him, but Jack angrily says that his daughter is still missing. Mannan then bitterly remarks that it was karma for what Jack had done during his time as a hitman. He later watches as Jack opens up to Rossi about what his former life cost him. Then, Katie's father enters the room and asks to apologize to Jack alone; Mannan reluctantly leaves with Reid and Rossi. Unbeknownst to them, the man hires Jack to track down and kill Lindsey and Katie's remaining abductors, who he identifies to Jack. As a result, Jack escapes the police station, assaulting Mannan and stealing his car. In the end, Jack rescues Lindsey and kills the group's leader. Mannan is last seen meeting back up with Jack and Lindsey and escorting them away.