When asked about the character model of CJ, actor Young Maylay stated that the development team took "very professional" photographs of him to model CJ.
Unlike the principal characters of previous Grand Theft Auto games, CJ's appearance is highly customisable, as the player can purchase hair cuts, tattoos and clothing for him. Certain clothes, tattoos and hairstyles improve CJ's standing with his fellow gang members as well as his sex appeal to his selective girlfriends. As CJ rides bikes, drives cars and motorcycles, and flies aircraft, his skill will improve in each. The player can also choose to exercise, which improves skills such as CJ's muscle and stamina.
Carl Johnson's personality markedly differs from the previous playable protagonists in the Grand Theft Auto series; while Claude and Tommy Vercetti are both depicted as sociopathic and feeling no regret for the murders that they commit, CJ is depicted as having a considerably less violent personality, occasionally giving his victims a chance to redeem themselves (one example being his failed attempt to convince Eddie Pulaski to leave Frank Tenpenny's side). Furthermore, CJ also displays genuine remorse for having to kill fellow Grove Street members Ryder and Big Smoke, who he had previously considered two of his closest friends. However, he still has no problem and feels no regret with killing members of other gangs, and willingly murders anyone that gets in his way of reclaiming Grove Street Families turf or try to sabotage his businesses, though the gangsters are also trying to kill CJ in return. CJ's naïve personality, inexperience, and face-value interpretations of other characters' responses occasionally leads them to question CJ's intelligence (notably The Truth and Catalina).
In 1987, five years prior to the events of the game, CJ causes the accidental death of his brother, Brian. Soon afterwards, CJ decides to leave his gang life behind by moving to Liberty City, where he begins working with Joey Leone in the car theft business.
The game's main storyline begins with CJ's return to Los Santos following the death of his mother, Beverly Johnson. Upon his arrival, CJ is confronted by Officers Frank Tenpenny, Eddie Pulaski and Jimmy Hernandez, three members of C.R.A.S.H. (Community Resources Against Street Hoodlums). Tenpenny and his associates warn CJ early on that they intend to frame him for the murder of police officer Ralph Pendlebury, whom C.R.A.S.H. killed to prevent him from exposing their illegal activities. They also force CJ to work for them in exchange for his safety and the safety of his family and friends.
After his confrontation with C.R.A.S.H, CJ returns to Grove Street where he is reunited with his siblings, Sweet and Kendl Johnson and his childhood friends, Big Smoke and Ryder. CJ learns that his gang, the Grove Street Families lost much of their power and influence to their rival gangs, in particular, the Ballas, during his absence. Shortly after his return, CJ aids his allies in re-establishing the Grove Street Families' dominance by expelling crack dealers, acquiring weapons, and regaining lost gang territory. CJ also aids his friend OG Loc in jump starting his rapping career (destroying the career of successful rapper Madd Dogg in the process) and is later introduced to Kendl's boyfriend Cesar Vialpando, the leader of the Varrios Los Aztecas street gang.
However, the Grove Street Families' resurgence is short lived as CJ discovers that Big Smoke and Ryder have betrayed the gang by forming alliances with C.R.A.S.H. and the Ballas. Sweet is ambushed by a group of Ballas on that same day and is wounded in the subsequent gunfight. CJ arrives on the scene and saves his brother's life but both are arrested by the police. Sweet is tried, convicted of several felonies and sentenced to life in prison whilst CJ is taken by C.R.A.S.H. and left in the countryside. In the aftermath of these events, the Grove Street Families lose their power and influence once more and subsequently relinquish all of their territory to their rival gangs.
During his time in the countryside, CJ is introduced to "The Truth", he aids Cesar's cousin Catalina in performing a number of robberies and he takes part in two illegal street races where he meets and befriends Wu Zi Mu. After his brief stay in the countryside, CJ and his associates head north for San Fierro, where they establish a vehicle chop shop and dealership. CJ also works for the local Triads (strengthening his ties with Wu Zi Mu and Ran Fa Li in the process) and infiltrates and destroys San Andreas' largest drug cartel, the Loco Syndicate. With some help from Cesar and The Triads, CJ also exacts revenge on his former friend Ryder by killing him during his meeting with the Loco Syndicate.
Following the destruction of the Loco Syndicate, CJ is contacted by one of its members Mike Toreno, now revealed to be an undercover government agent. CJ begins working for Toreno in exchange for Sweet's release from prison and subsequently ventures into the desert and Las Venturas. During this time, CJ acquires a pilot's license, he aids Wu Zi Mu in running the Four Dragons Casino, and he saves Madd Dogg's life by intervening in his attempted suicide. CJ also kills Eddie Pulaski, who had just killed fellow officer and C.R.A.S.H member Jimmy Hernandez. At the conclusion of these events, CJ and his associates begin preparing for their return to Los Santos.
Upon his return to Los Santos, CJ regains control of Madd Dogg's mansion and is later reunited with Sweet after his release from prison. However, Sweet refuses to leave their gang life behind and talks CJ into re-establishing the Grove Street Families once more. Together, they take back Grove Street and begin slowly rebuilding their gang by removing drug dealers and drug addicts from the streets. Meanwhile, Tenpenny is tried for his crimes but all charges are dropped due to a lack of evidence, resulting in a city-wide riot. In all the chaos, CJ aids Cesar in re-establishing his gang, the Varrios Los Aztecas, before confronting (and ultimately killing) Big Smoke at his crack palace. Afterwards, CJ pursues Tenpenny in a thrilling car chase which ultimately ends with Tenpenny crashing in front of the Johnson House. CJ prepares to shoot Tenpenny, but Sweet stops him as Tenpenny is already dying from his injuries and shooting him would leave evidence behind. With Tenpenny now dead, any loose ends in CJ's life are resolved and the city returns to its normal state.
At the conclusion of the game, Madd Dogg visits the Johnson house and announces that he has received a gold record for his new album. Everyone inside discusses how they must now shift their focus to maintaining the gang and their business ventures and otherwise keep a low profile. CJ then gets up and leaves the house. Kendl then asks where he's going, and he replies, "Fittin' to hit the block, see what's happening," as the game's main storyline ends.
Voice actor Young Maylay stated that he was influenced by his own life when portraying CJ. "[The development team] wanted authentic L.A., that's where I'm from and they knew that, so that's what I gave 'em," he added. "I put Maylay on CJ. I make him as much me as I can, without to [sic] much changing of the script."
The character of Carl Johnson received critical acclaim after the release of Grand Theft Auto: San Andreas, managing to become included in many lists of the best characters in video games. He was included on Jesse Schedeen of IGN's list of Grand Theft Auto Favorite Badasses. They said, "Of all the protagonists in all the GTA games, few are as compelling or flat out badass as Carl "CJ" Johnson," and also went on to praise the character customisation and available assets. Crave Online's Paul Tamburro also placed Carl eighth on their Top 10 Most Memorable GTA Characters, stating that "it was refreshing to take control of a character who was considerate about when and when not to commit wanton mass-slaughtering." Matthew Cooper of Sabotage Times placed the character in his list of the top 10 characters in the Grand Theft Auto series, stating what stands out him from other central characters is the fact "he was the first to appear with a conscience, the first that didn't seem to enjoy killing copious amounts of people."
GameDaily listed Carl among their list of the best black characters in video games, refusing the idea that he reinforces negative stereotypes since he is "more ghetto-born James Bond than straight-up gangsta". Similarly, Larry Hester of Complex Gaming placed Carl second on his list of the 10 Best Black Characters in Video Games, naming him the "gangbanger with a good heart." Carl was also placed 77th on GamesRadar's list of the 100 Best Heroes in Video Games, quoting that "few [Grand Theft Auto] heroes have been as charismatic as him, and few likely will in the future." UGO Networks have placed Carl as the second character who most deserves his own live-action film.
In 2008, The Age ranked Carl as the 33rd greatest Xbox character of all time, noting he as "the most humble" Grand Theft Auto anti-heroes, and as "one of the first strong African-American lead characters in any major videogame." In 2012, GamesRadar placed Carl 77th on their list of "most memorable, influential, and badass" protagonist in games, saying that "Few GTA heroes have been as charismatic as him, and few likely will in the future." Although Carl ultimately did not make the cut, Game Informer staff considered his inclusion in their "30 characters that defined a decade" collection, with Matt Helgeson saying, "He could have easily been another gangster stereotype, but by the end of San Andreas we see CJ as a flawed, but ultimately good man who did the best he could in the worst of circumstances."