Mason Gilroy is a psychotic former MI6 spy/freelancer who appears as a recurring character in the third season. He is portrayed by Chris Vance. Described by Michael as, "Someone who arranges coup d'etat for a living."
After Strickler is killed, a mysterious figure comes into town in order to "clean things up." This cleanup includes offing Michael's CIA contact, Garcia, among other threatening acts. Michael, with Sam's help, is able to piece together who he is when they realize the threatening acts are actually re-enactmemts of past crimes in which Gilroy was suspected but never confirmed as the culprit. Michael is approached by Gilroy to join him on a "special job" but is hesitant to hire Michael until he "gets to know him better," which is Gilroy talk for deciding whether or not Michael is as much of a psychopath as him. After witnessing some of Michael's work first hand, he is impressed and decides to hire Michael.
After performing some errands for Gilroy and doing some investigative work of his own, Michael discovers that Gilroy's operation in Miami involves a plane heading from Chile to Poland. The plane is transporting an extremely high-risk prisoner, and it is implied that Gilroy wants to intercept the plane. After diverting the plane from Miami International Airport (by using a fuel truck that caught fire to shut down flight operations), he was paid 10 million dollars for securing the release of the prisoner. By the time Michael arrived, Gilroy had been shot in the abdomen, strapped to an explosive device, and handcuffed to the steering wheel of his vehicle. After briefly speaking with Michael, warning him about the present dangers, "He paid me ten million dollars, apparently his plan was to shoot me and come after you (and ruefully acknowledging that he himself had been manipulated and betrayed)," Gilroy dies when the explosive device detonates and the vehicle explodes.
Mason Gilroy was a former spy for MI6 who freelanced all over the world and he also worked with Tom Strickler before Strickler was shot dead at the hands of burned spy, Michael Westen. He is also responsible for killing Michael's CIA point of contact, Diego Garcia by "...Arrange[ing] to have him fall out of a fourth story window."
A freelance psychopath, Gilroy has no problem killing innocent people and rather enjoys it, going as far as to kill his own teammates if they fail during a mission (Gilroy informs Michael that a thief partner ("Claude") died due to "complications from his injury", where he only broke his ankle). He is rather good at what he does, committing violent acts and killing people without getting caught or being blamed for it. Though he has been a person of interest.