Reformed Criminal

Once upon a time that guy lived on the wrong side of the law. Whether a thief, a highway robber, or other outlaw, he had a Heel Face Turn and through various circumstances, decided to reform his ways. Now he puts his experience as a former criminal to the service of the law, as a Reformed Criminal.

Compare Recruiting the Criminal, when the character works for the law while still remaining a criminal, and Boxed Crook, when he's coerced or blackmailed into cooperating with the law. It may even become The Atoner in some cases.

Examples of Reformed Criminal include:

Anime and Manga

Kotobuki, of Tsubasa Those With Wings, at the very beginning.
Though they weren't particularly hardened criminals, Fate and every member of the Wolkenritter (Except for Rein, who didn't exist at that point in the storyline) of Magical Girl Lyrical Nanoha started out as villains who were arrested by the TSAB, sentenced to work for the TSAB for a period of time as their punishment, and then decided to continue working for them full time once their sentence ended.
- In fact, it emerged in StrikerS that Hayate still sometimes gets flak from being at the heart of the incident, despite it having taken place a decade in the past. She works as hard as she does in a bid to shake off the stigma.
Kurama and Hiei of Yu Yu Hakusho started out as criminals before they each had a Heel Face Turn.

Comic Books

In the Marvel universe, Scott Lang, who becomes the second Ant-Man, is a former burglar.
Also in Marvel, the Thunderbolts started out as a group of villains only PRETENDING to be superheroes... until they decided they liked being heroes more and became ACTUAL heroes.
Most of the Flash's villains, the Rogues, have reformed over the course of years; only Trickster (before his death) and Pied Piper have remained.
The Sandman (no, not that one) tried to be a good guy for awhile. It didn't take.
Steeljack from Astro City.
The Garth Ennis mini-series Pride and Joy is about a reformed criminal trying to leave his past behind him, even though he got away with murder.
Max Damage from Incorruptible
Both Marv and Dwight in Sin City admit to commiting crimes in the past but it's usually unclear what they did.
- In Just Another Saturday Night, Marv finds a group of rich kids setting winos on fire and kills them all. He also makes reference to "bar fights". His characterization is based on a love of violence, but only toward people who have it coming.

Film

Gan Xing from Red Cliff (based on Gan Ning from Romance of the Three Kingdoms) started out as a pirate, but he and his men ended up joining the regular army.
In the Spaghetti Western They Call Me Trinity Trinity's brother, who is/was a criminial, is working as the sheriff of the town.
Carlito of Carlito's Way.
Axel Foley from Beverly Hills Cop, who regularly notes that he "wasn't always a cop".

Literature

The sidekicks of Judge Dee tend to be reformed highway robbers and con men. Knowing the criminal underworld from the inside makes them quite useful to the magistrate's investigations.
In Years of Rice and Salt, Kheim, the admiral of the Ming fleet that discovers America, is a former pirate.
In one or two of the Monsieur Lecoq books by Emile Gaboriau, the great detective Lecoq (to some degree based on Vidocq) is said to be an ex-criminal.
Discworld: In Going Postal, Making Money, and Raising Steam, Moist Von Lipwig, a con artist, is recruited to handle the post office, the Ankh-Morpork mint, and the new railroad, respectively.
In the 1632 story universe, the Grantville Gazette short stories written about the downtime NCIS (the stories are explicit homages to the TV show of the same name) have more than a few reformed criminals in the service, including the main male protagonist.
In the Warrior Cats series, Blackstar. After doing things against the warrior code (stealing kits from another Clan, killing other cats needlessly), he lived as a rogue for a while, but eventually rejoined the Clan, became its leader, and hasn't done anything like that since.
In the Resident Evil novelizations by S.D. Perry, Jill Valentine is said to have been a thief, following on the footsteps of her father Dick Valentine, but eventually gave up and joined the S.T.A.R.S. police unit (which incidentally explains her mastery of lockpicks).
Molly, of The Dresden Files, is this. Breaking the Laws of Magic is a biiiig no-no.
- Harry himself is considered this by most of the Wizard population. (He was innocent; DuMorne shot first.) Now he's the regional commander of the Wardens. Go figure.
  - In both those examples, being taken under the wing of a more senior wizard can reduce the sentence from immediate execution to one-chance probation before the same. For both the accused and the wizard who takes them in. The thought is, if they can invoke this trope, both of them are eventually free of the probation, but if not, Black Magic (which actually does corrupt) must be stifled.
- Sanya used to be the host of a Denarian. Now he's a Knight of the Cross. In fact, it's the job of the Knights to invoke this for the Denarians' hosts.
- Lash. Doubles as a Tear Jerker.
- Bob, actually. But this has more to do with who his owner is at any time. When Harry picked him up, he got modelled after Harry's teenage self. Before? Warden DuMorne. Before him? Freaking arch-necromancer Heinrich Kemmler.
- Thomas has shades of this, by trying to not hurt humans with his Demon.
In the Codex Alera, Fade is this. Actually, he's Araris Valerian guarding Tavi and Isana. Tavi later causes this in Fidelias Ex Cursori.
In the Sword of Truth, Richard causes this in just about everyone he meets. The Sisters of the Light, Nicci, the Mord-Sith, D'Hara in general, the people of Altur'Rang, the list goes on and on. The D'Haran commanders lampshade this.
- Denna, oh so much.

Live-Action TV

Both of the hosts of the Discovery Channel show It Takes a Thief
And the ABC 1968-70 TV series It Takes a Thief, although in that case the reform was forced from without.
Kaz, about a petty thief who became a lawyer while serving a prison sentence.
In White Collar, Neal Caffrey, the show's main character, was formerly a con artist before he became a consultant for the FBI in order to keep out of prison
In Power Rangers SPD, Jack and Z were Just Like Robin Hood thieves until the Space Police caught them, and the commander offered them the chance to serve as policemen (and Power Rangers) as an alternate to a prison term. Z takes to it immediately; Jack is more reserved but stays legit even after leaving the force.

Video Games

At the end of the third Ace Attorney game, former Gentleman Thief Ron DeLite, aka Mask DeMasque says that he's starting a security company to help businesses defend themselves against thieves. And also that he's going to sell the security plans to crooks. At least he's not stealing anymore?
Before his appearance in StarCraft games, James Raynor used to be a bandit, until he nearly got caught (his partner took all the blame, and was sentenced to prison), then he settled down and became a sheriff of Mar Sara colony.
After the gang left him in Red Dead Redemption, John Marston gives up the criminal life and chooses to spend time with his family and work on his ranch. Unfortunately, Edgar Ross doesn't see it that way.

Western Animation

Kevin Levin, once a troubled (but still deadly) kid turned arms dealer, is convinced as a teenager to fight for good after a Plumber (meaning a member of the secret alien police) saves his life at the cost of his own.
Tohru used to be a part of the Dark Hand before his Heel Face Turn at the end of Jackie Chan Adventures.
Rubberband Man in Static Shock.

Real Life

A famous Real Life example is Vidocq, a French thief who ended up chief of police, and later became the first private detective ever.
Frank Abagnale Jr, both in real life and in the film Catch Me If You Can, went from passing millions of dollars worth of fraudulent checks to working as a fraud consultant for the FBI.
Several hackers (convicted and otherwise) have found gainful employment in the computer security field.