Sergeant Colin Sullivan is an undercover gangster within the Boston Police department, acting as the personal mole for the secondary main antagonist, ruthless mob boss Frank Costello. Colin serves as the secondary later true main antagonist in Martin Scorsese's crime drama The Departed. He is the counterpart to the film's protagonist, undercover cop Billy Costigan.
Colin was a little boy when he met mob boss Frank Costello. He soon took Sullivan under his wing. 20 years later Sullivan graduates from the Boston police school but what they don't know is that Sullivan is working for the Costello as a informant. But little did he know that the police also planted an informant in the mob. He then starts a relationship with Madoyln Madden who Bill Costigan (the undercover cop) is also seeing under terms of his probation. After Costello escapes a sting he realizes that there might be police mole. Sullivan becomes determined to find to find out the identity of the rat. One night in a porn theater, Sullivan tells Costello to gather his gang's Social Security numbers so he could find the informant. Later, Costigan and Sullivan see each other and Costigan chases Sullivan. Sullivan hides from but then stabs someone because he thought it was him, only to realize he stabbed a Chinese Restaurant worker. Sullivan then learns that Costello is also an FBI informant. Sullivan confronts Costello after he foiled a drug shipment pickup by Costello. Costello says he only gave up people who were already going down. Sullivan asked him if the FBI knew about his connection to Costello. Costello denied it, saying he would never give him up, because he was like a son to him. Sullivan inquired if that was all it was about, all the murder, sex and "no sons", suggesting Costello was sterile. Enraged, Costello shoots at Sullivan, who fires back and kills him. When he gets back to the police station he is hailed as a hero for killing Costello. But looking though files Costigan learns that Sullivan is a mole. Costigan then sends a message to Sullivan to meet him on the roof of the old mill. There he arrests Sullivan but as they go down the elevator the doors open and then Costigan is shot in the head by a cop, Barrigan who later shoots Brown, the cop that Costigan trusted. Sullivan then learns that there is two moles in the police, him & Barrigan. Sullivan then kills Barrigan while he is not looking. He goes home after Costigan's wake but then he is confronted by Dignam, a cop who resigned after the murder of his boss, who while a very unpleasant man, sought to avenge the murders of his boss and Costigan and saw no other way to do it as the jury would always believe Sullivan, a respected police officer, over him, a bad mouth citizen (as he is now resigned). Sullivan, realizing this was the end, simply said "Okay" before Dignam fatally shot him in the head and left.
- Colin Sullivan is based on FBI agent John Connolly, who protected and tipped off Boston Irish mobster James "Whitey" Bulger, who is the basis for Frank Costello.
- Both Barrigan and Sullivan are possibly a tribute to "Mafia Cops" Louis Eppolito and Stephen Caracappa, two Italian American NYPD officers who worked on behalf of the New York Mafia.
- Sullivan is portrayed by award winning American actor Matt Damon, who was born and raised in Cambridge, a bit north of Boston, Massachusetts.
- He was based off of Inspector Lau Kin Ming from Infernal Affairs.