He is portrayed by Jack Nicholson.
At a young age, Colin Sullivan is introduced to organized crime through Irish mobster Frank Costello in the Irish neighborhood of South Boston. Costello trains him to become his mole inside the Massachusetts State Police. Sullivan is accepted into the Special Investigations Unit, which focuses on organized crime. Before he graduates from the Police Academy, Billy Costigan is asked by Captain Oliver Queenan and Staff Sergeant Sean Dignam to become an Undercover Agent, as his childhood and family ties to organized crime make him a perfect infiltrator. He drops out of the Academy and does time in prison on a fake assault charge to increase his credibility.
As both infiltrate their respective organizations, Sullivan begins a romance with psychiatrist Madolyn Madden. Costigan sees her for his probation, part of the undercover role, and also develops a relationship with her as well. After Costello barely escapes a sting operation, both moles become aware of the other's existence, though not their respective identities. Sullivan is told to find the "rat" and asks Costello for his underlings' personal information (birthdates, Social Security numbers, etc.) to determine who is the informant within Costello's crew. Costigan follows Costello into a pornographic movie theater where Costello gives Sullivan an envelope with the details. Costigan then chases Sullivan through Boston's Chinatown district. When it is over, both men still do not know each other's identity. Sullivan then has Queenan tailed as he meets Costigan in an abandoned building. Sullivan sends Costello's men in and Queenan distracts them to let Costigan escape. He is thrown from the roof and dies at the feet of Costigan. When the mobsters escape, Costigan pretends he has come to rejoin them. The trailing State Troopers open fire on Costello's crew, which causes casualties on both sides. Later, at one of Costello's safe houses, a mortally wounded Delahunt, one of Costello's enforcers, tells Costigan "I gave you the wrong address, but you showed up at the right one." He then asks him if he knows why he didn't say anything about it before dying. When Costello's henchman "Fitzy" does an unsuccessful job of hiding the corpse, it is revealed that Delahunt was also an undercover police officer. Costello berates the rest of his crew for believing this, knowing that the more likely explanation is that the police declared Delahunt an informant in an attempt to stop Costello himself for continuing his search for the mole.
Now under scrutiny, Sullivan is attacked by a suspicious Dignam because of Queenan's death. Captain George Ellerby places Dignam on leave. Using Queenan's phone, Sullivan calls Costigan, who refuses to abort his mission. Sullivan learns of Costello's role as an informant for the FBI from Queenan's diary, causing him to worry about his dual identity being revealed. Sullivan calls Costello to inform him he has a tail as he and his men drive to a cocaine drop-off. Costello furiously demands that Sullivan gets rid of the tail, to which he agrees. Fed up with Costello's arrogance, along with the knowledge that he is an FBI informant, Sullivan gets rid of the tail and convinces Ellerby to a special ops strike at the drop-off where he intends to confront Costello. With Costigan's help, Sullivan tracks down where the drop-off is located. Costello and his crew become trapped in a gunfight with the police, resulting in most of the mobsters being killed. As Costello attempts an escape he is confronted by Sullivan. Costello admits he is an occasional FBI mole. Sullivan rages at him and then asks if the FBI knows about his connection to Costello's crew. Costello denies ever giving up a man who was like a son to him. Outraged, Sullivan asks Costello if that was all it was about; all the murder, sex and "no sons" suggesting Costello was sterile. Angered, Costello shoots at him but misses and Sullivan fires back, hitting him multiple times. Near death, Costello tries to shoot Sullivan again, but Sullivan dodges the shot and returns fire, killing Costello.