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."
Michael figures out who he is when Gilroy re-enacts his crimes to Michael. He is approached by Michael to join him on his "secret job" but turns Michael away, saying he is not good enough for his job, but after seeing Michael pull off a job 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.