Season two

Colvin is at the scene when a child is accidentally shot during a turf war, appalled at the senselessness of the killing. When ordered to crack down on the area, his second in command, Dennis Mello, states that they waited too long to make the arrests they had. Colvin begins to question if they are actually doing their job.^[1]

Season three

Colvin, months away from being eligible to retire on a major's pension, decides to make a last effort to have a real impact on the community. He recognizes that much of his time and resources are spent on policing addicts and low level dealers, which never improves the situation in his district and leaves little time for "real" police work. All of Baltimore's district majors are under extreme pressure to reduce the city's violent crime rate from Mayor Clarence Royce, who is seeking re-election.

After Commissioner Ervin Burrell relieves Major Marvin Taylor from his post commanding the Eastern District due to his poor performance, other district commanders begin "juking" their stats to make crime rates appear to drop. Colvin refuses to do this, and his stats honestly reflect a 2% rise in felonies. He is quickly chewed out by Deputy Commissioner William Rawls, while Burrell threatens to have him replaced.

Colvin wonders if there is a way for low-level users to take drugs safely without facing punishment. After the attempted murder of Kenneth Dozerman, Colvin decides that he will independently set up three "free zones" in the Western where addicts and dealers can conduct their business under supervision, but without interference. This moves the drug trade into a controlled, uninhabited area to protect the rest of his district.

Colvin does not seek the permission or approval of his superiors before implementing his plan, and ignores the concerns of his subordinates, including Mello and Ellis Carver, who are charged with ensuring no violence takes place within the free zones. One of these areas becomes known as "Hamsterdam", after Amsterdam's liberal drug laws. Because his retirement is imminent and he is guaranteed a pension, Colvin believes he'll be free from any consequences should his plan be discovered. Legalizing drugs in Hamsterdam allows him to redirect police resources to quality felony cases elsewhere.

After implementing the Hamsterdam plan for five weeks, Colvin delivers a cumulative 14% reduction in the felony rate, unheard of in the Western's history. Colvin is forced to take his vacation time immediately after revealing his experiment to Rawls and Burrell. Royce considers trying to spin Hamsterdam as an enforcement strategy because of its success in lowering the crime rate. However, in the meantime, Herc leaks information about Hamsterdam to the press after rejecting Carver's request to help him move a homicide victim off the free zone premises.

After realizing that public opinion is sharply against the free zones, and that there are broader political ramifications (the White House's Deputy Drug Czar threatens to pull all Federal funding from the city if the entire endeavor isn't immediately ended and publicly negated), Royce recants and decides to end the Hamsterdam experiment. Burrell and Rawls force Colvin into becoming a scapegoat by threatening to persecute his officers. He is demoted to lieutenant and thus gets a lowered pension. Burrell also contacts Johns Hopkins University and convinces them to withdraw a job offer for their campus security force.

As a commanding officer, Colvin is well liked by his men. Colvin has a major impact on Carver, convincing him to reassess his role as DEU sergeant and to take a more community-minded approach to policing. He also reconnects with Jimmy McNulty, who had started out as a beat officer under his command. Colvin's last piece of detective work involves McNulty's Major Case Unit: Stringer Bell contacts Colvin to inform on Avon Barksdale, and Colvin passes the information on to McNulty. In Colvin, Stringer sees a fellow reformer who feels his superiors are preventing useful work from being done. As Bell puts it, they are "both trying to make sense of this game," though from opposite sides of the law.^[2]

Season four

Colvin attempts to supplement his diminished pension by working as the head of security for a downtown hotel. He becomes disillusioned with the post when the hotel manager refuses to let him arrest a wealthy client who had assaulted a prostitute in his hotel room, and quits the job soon thereafter. Later, he is approached with another job from The Deacon, who has learned of a large grant to the University of Maryland to look at repeat violent offenders.

The study is led by Dr. David Parenti. Colvin has developed a reputation among academics as a result of his Hamsterdam experiment, and the Deacon helps him secure a job offer with Parenti as a field researcher. Parenti initially plans to focus on 18- to 21-year-olds, but Colvin convinces Parenti to look at Edward Tilghman Middle School for his target group.

Colvin identifies for Parenti the two types of West Baltimore students: "stoop" kids who obey their parents' instructions and who are respectful of authority, and "corner" kids, the kids who sell drugs, disrupt class, and are aspiring gangsters disrespectful of authority. Together, they isolate ten corner kids into a classroom where Parenti and a UM doctoral student study them while Colvin acts as the mediator. In this classroom, no suspensions are handed down to misbehaving students, as it is seen as a punishment the students often utilize to get out of class intentionally.

Colvin begins to take an interest in Namond Brice, one of the most disruptive students. He allows Namond to stay at his home when Carver arrests him for selling drugs and his mother is out of town. Colvin takes 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 and realizes he will only get himself killed or arrested if he remains in his current household.^[3]

Colvin then talks with Wee-Bey Brice, Namond's incarcerated father, explaining Namond could 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 to be living with Colvin and his wife at the conclusion of season four.^[3]

Season five

Colvin appears briefly with a gray and white goatee, attending Namond's high school competitive debate. He looks displeased when Mayor Tommy Carcetti visits the event, using it to burnish his political image. Outside the debate, Carcetti approaches Colvin and apologizes for being unable to support the Hamsterdam experiment, saying no politician could run with the idea politically despite hinting at the time that he supported the initiative. Colvin refuses to shake Carcetti's extended hand, and leaves the Mayor ashamed and speechless when he bluntly says "well then, Mr. Mayor, I guess there's nothing more to be done".