Very little is known about Mark's past, other than he founded and currently owns a law firm. He also struck up a good friendship with Jesse Gentry, a paralegal.
When Jesse is arrested under the mistaken identity of his twin brother Wallace Hines, he hires Mark to represent him. Once the mistake is discovered and Jesse is released, Mark warns Hotch and Sgt. Johnson that he will file a lawsuit against the FBI if they make even the slightest move that could be taken for harassment towards Jesse, diminishing Hotch's concerns about the threat that Wallace may pose to Jesse after learning they are twins. Later, Carla is taken to the precinct by the BAU for questioning; she is allowed to leave after they finish, but a tail is put on her. When she evades the police, Garcia finds that she had contacted Mark and he had helped her to accomplish that.
The BAU marches into the firm, confronting him. He, at first, tries to use the attorney-client confidentiality privilege to justify his actions, but Hotch and JJ threaten him with accusing him of accessory to murder, as they can prove Jesse is a killer too; his last try to avoid them is squandered when Hotch promises to parade him in cuffs in front of "all those cameras you love so much". Mark quickly loses the attitude, affirms he doesn't know where Carla or Jesse are, but offers his cooperation. Mark is last seen providing important information about how Carla set up a trust fund for Jesse, pointing them in the direction of Jesse and Wallace's father, a schizophrenic man named Bill Robbins. As he couldn't have know at the time that Jesse had become a killer, and what he helped Carla with isn't a crime, he probably was released without charges.