Colvin joined the Baltimore Police Department in 1974 (according to Season 3, he had 30 years on) patroling his home neighborhood in the department's Western District; one of his early posts was at Pensey and Fremont. Over his tenure, he advanced to the rank of major of the Western District. Colvin's philosophy of policing involved protecting the community he served by making quality arrests through the use of trusted informants on his foot post. As a commander he insisted that his men learn their sense of direction, their foot post, and urged them to focus on doing real police work. Toward the end of his career, he began seeing the war on drugs as an ineffective waste of time and resources in his district that brought about too many unnecessary deaths.
Colvin was first seen as a Major and commander of the Western district in season two. He attended the scene of the accidental shooting of a child during a drug turf war and was appalled at the senseless nature of the incident. When ordered to crack down on the area, his second in command Dennis Mello stated that they waited too long to make the arrests they had while Colvin begins to question what it is they are really doing on their job.
In season three, Colvin was nearing retirement and decided to make a last effort to have a real impact on the community he had been policing for thirty years. He recognized that much of his time and resources were spent on policing addicts and low level dealers, which never seemed to improve the situation in his district and left little time for "real" police work. All of Baltimore's district Majors were under extreme pressure from the mayor's office to reduce the city's violent crime rate in preparation for the mayoral primary campaign. After Commissioner Ervin Burrell relieved Major Taylor of his post as the Eastern district commander for his poor performance, every other major began "juking" their stats to make crime rates appear to drop. Colvin refused to participate, and his stats honestly reflected a 2% rise in felonies. He was quickly berated by Deputy Rawls and his command post was threatened by Commissioner Burrell who claimed "I don't care how many years you have on this job, if the felony rates don't fall, you most certainly will."
Colvin wondered if there was a way for drugs to be made safe for low level users to take them without facing punishment; comparing the city's drug problems to the illegal public consumption of alcohol, which was circumvented when people began keeping their beer in a paper bag. After the attempted murder of Officer Dozerman, Colvin finally decided that he would independently set up three "free zones" in his district where addicts and dealers were allowed to conduct their business under supervision but without interference. This would move the drug trade into a controlled, uninhabited area to protect the rest of his district. Colvin did not seek the permission or approval of any of his superiors before implementing his plan, and ignored the concerns of his subordinates Lieutenant Dennis Mello and DEU Sergeant Ellis Carver, who were charged by Colvin with ensuring no violence took place within the "free zones." One of these areas became known as "Hamsterdam", after Amsterdam's liberal drug laws. Because his retirement was imminent and he was guaranteed a major's pension, Colvin believed he would be free from any consequences should his plan be discovered. Although his project initially drew suspicion from the district's dealers, he convinced them to move their trade by brutally cracking down on any drug dealing outside of Hamsterdam. Colvin was concerned when he found a single retiree still living in one of the houses in Hamsterdam. He had to plead Colonel Foerster to have the old woman relocated to new housing, lying to both her and the Colonel about the reasons why.
As a commanding officer Colvin was well-liked by his men. Colvin had a significant impact on Ellis Carver, convincing him to reassess his role as DEU sergeant and to take a more community-minded approach to policing. Colvin also reconnected with Jimmy McNulty, who had started out as a beat officer under his command. Colvin's last piece of detective work involved McNulty's major case unit — Stringer Bell contacted Colvin to inform against his partner Avon Barksdale and Colvin passed the information on to McNulty. In Colvin, Bell had seen a fellow reformer who felt his superiors were preventing useful work from being done.
Colvin was forced to take his vacation time immediately after revealing his experiment to the department's upper command. The city's politicians considered trying to spin Hamsterdam as a public health measure but eventually decided it was too dangerous. With the free zones now public knowledge, Commissioner Burrell offered Colvin to the mayor as a scapegoat. Commissioner Burrell and Deputy Rawls threatened to persecute his officers whenever they could for whatever reason they could if he did not leave the department at a lower rank, with a reduced pension. To protect his men, Colvin grudgingly accepted. Burrell made things even worse for Colvin having contacted Johns Hopkins University which had agreed to hire him as their deputy commander of campus security upon his retirement. Burrell personally informed them of his actions involved with "Hamsterdam" and they withdrew Colvin's job offer.
As season four began, Colvin was supplementing his (diminished) pension by working as head of security for a downtown hotel. Colvin became disillusioned with the post when the hotel manager refused to let him arrest a wealthy client who had savagely beaten a prostitute in his hotel room. He left the job soon afterwards.
Colvin was approached with another job by his friend, The Deacon. The deacon had learned of a large grant to the University of Maryland School of Social Work to look at repeat violent offenders. The study was led by Dr. David Parenti. Colvin's reputation among academics as the man who attempted to legalize drugs in Western Baltimore secured him a job offer as a field researcher. Parenti initially planned to focus on 18-to-21-year-olds, but following an interview with an extremely hostile and violent boy in that age group, Colvin sensed that they would have to look at a younger group to effect any change. He convinced Parenti to look at Edward Tilghman Middle School for his target group.
Colvin identified for Parenti the two types of West Baltimore students: "stoop" kids, the kids who obey their parents' instructions to stay on the stoop or front steps of their house, go to school, and are respectful of authority; and "corner" kids, the kids who sell drugs on the corner, disrupt class, and are aspiring gangsters disrespectful of authority. Together, they isolated ten corner kids into a classroom where Parenti and UM doctoral student Miss Duquette studied them while Colvin acted as the mediator.
Colvin began to take an interest in Namond Brice, one of most disruptive students. He allowed Namond to stay at his home when Carver arrests Namond for selling drugs, and his mother was out of town. Colvin took him home the next day and sees first hand that his mother is pushing him into drug dealing. After seeing how Namond has progressed in school, Colvin sees Namond's potential. He realizes that Namond was never fit for the corners and will only end up being killed or in jail if he remains in his current household. Colvin then talks with Wee-Bey Brice, Namond's incarcerated father, claiming that Namond can have a life outside of West Baltimore given the proper support from Colvin and his wife. After thinking it over, Wee-Bey tells Namond's mother to send him to live with Colvin as he wants him to have a future. Namond is seen living with Colvin and his wife at the conclusion of season four. 
|Season 2 appearances|
|"Ebb Tide"||"Collateral Damage"||"Hot Shots"||"Hard Cases"||"Undertow"|
|"All Prologue"||"Backwash"||"Duck and Cover"||"Stray Rounds"||"Storm Warnings"|
|"Bad Dreams"||"Port in a Storm"|
|Season 3 appearances|
|"Time after Time"||"All Due Respect"||"Dead Soldiers"||"Amsterdam"||"Straight and True"|
|"Homecoming"||"Back Burners"||"Moral Midgetry"||"Slapstick"||"Reformation"|
|"Middle Ground"||"Mission Accomplished"|
|Season 4 appearances|
|"Boys of Summer"||"Soft Eyes"||"Home Rooms"||"Refugees"||"Alliances"|
|"Margin of Error"||"Unto Others"||"Corner Boys"||"Know Your Place"||"Misgivings"|
|"A New Day"||"That's Got His Own"||"Final Grades"|
|Season 5 appearances|
|"More with Less"||"Unconfirmed Reports"||"Not for Attribution"||"Transitions"||"React Quotes"|
|"The Dickensian Aspect"||"Took"||"Clarifications"||"Late Editions"||"–30–"|