Before the events of The Mutation Situation, Jason was a human boy living in New York City. Concurrent to the events of that episode (but not revealed until Meet Mondo Gecko), he was skateboarding a half-pipe on his building rooftop when he was hit by one of the many canisters of mutagen the turtles accidentally dropped from the sky. He had last touched his pet leopard gecko Lars, and thus became a mutant gecko himself. He was rejected by his parents and kicked out, becoming homeless. Jason was taken in as an errand boy by the criminal Xever Montes, who was called "Mr. X" by his underlings. While employed by Xever, Jason was exploited into committing petty crimes including theft. Even so, Jason still enjoyed certain hobbies, including skateboarding as well as performing in the heavy metal band Savage Bliss.
Xever told Jason that the turtles were evil, and to help him capture them. In Meet Mondo Gecko, Jason met and befriended Michelangelo and Casey Jones while all three of them were skateboarding on building rooftops. The moment Mikey saw Jason, he named him "Mondo Gecko," which quickly became Jason's nickname.
As the three of them hang out, Casey nearly falls off a building until Michelangelo catches him, Mondo laughs with Casey angrily telling him that he could've gotten hurt, just as Mondo apologies, the three come across the Purple Dragons who are in the middle of stealing. When Mikey asks them what their doing, the Purple Dragons explain that their helping a friend move their valuables to their lair, Mikey doesn't think anything of it until Casey points out that their obviously lying. Mikey demands that they put the valuables back, Hun arrives, asking whose going to make them, as a fight breaks out between the Purple Dragons, Mikey and Casey, Casey is getting beat up by Hun while Mikey is beating up the rest of the Purple Dragons. At first Mondo hides until he is discovered by Hun and tries to fight him off but with little success, Mondo is kicked off the building by Hun where he is then saved by Mikey. After rescuing Mondo, Michelangelo takes out the other Purple Dragons, just leaving him and Casey up against Hun, while they fight against each other, it seems that Hun has the upper hand until he steps on a skateboard and falls into a trashcan at the bottom. While Mikey tied up the remaining Purple Dragons, Mondo then proceeded to taunt the Purple Dragons and smack them until he is held back by Casey who tells him that they have already won, Mondo thanked them for saving his life and tells them while on the streets it's everyone for themselves, where as Mikey tells Mondo that friends don't let each other fall off buildings. While Casey left his calling card on the Purple Dragons for the police, Mondo finds $30.00 on them and attempted to steal it until Mikey called him out on that. Mondo said that it's okay since it's from the bad guys where Casey demands that he puts the money back where it belonged. An argument broke out between Casey and Mondo over morality occurred with Mikey acting as a mediator, begging them to just go back to skating where Casey leaves saying he doesn't hang with criminals. Mondo yelled at Casey saying skating isn't a crime and then invited Mikey back to his place. Mondo leads Mikey to the sewers where the latter exclaims that Mondo lives in the sewers too, Mondo explains to Mikey that Mr. X has taken in a lot of mutants including himself in, while venturing into the sewers Mikey becomes trapped in a cage while trying to break free, Mikey tells Mondo to run, Mondo apologies and is revealed to be one of Xever's lackeys and that Xever is Mr. X. Mikey looks over to Mondo and asks him why has he done this when he thought they were friends. Mondo questions Fishface about whether the turtles were as bad as Xever says they are, Fishface tells Mondo that he shouldn't feel sympathy towards him for tonight Mikey is going to part take in a race to the death, surprised, Mondo tells him that is not part of the deal and that he thought it was just a normal race, Xever slaps Mondo. Mikey asks why Fishface is doing this, Fishface replies that it is simple economics, he brings mutants in to part take in races, he makes money and everyone wins, except for Mikey for Fishface explains that Mikey will lose, he tosses Michelangelo into a cellar with Casey who was also captured by Xever. Feeling bad for his friends being locked up Mondo attempted to free his friends Mondo appears and attempts to release them from the cell until he is found out from Xever, for Mondo's betrayal, Xever places Mondo in the race, while the lights come on it is revealed that some of Shredder's henchmen along with other members of the underworld are there to see the race, while introducing the contestants, Xever enters himself into the race, Casey states that if he can't beat giant fish in the race then he will retire his blades, Mondo tells him that Casey is wrong and that he has seen Fishface skate and panics where he is then smacked by Mikey and comes back to his senses. Baxter starts the race, during the race, Fishface reveals that he has rocket skates, going through trials that consist of electrified trashcan lids, an aquarium filled with piraña, giant boulders with spikes, arrows and closing doors, Casey gets caught between one door telling Mikey and Mondo to go on ahead to beat Fishface. Fishface releases oil on the ground, Mondo tells Mikey that Xever is going to win, Mikey doesn't believe that, through some teamwork with Mikey, Mondo and Casey, they manage to beat Xever over the finish line. Xever claims that Mondo and his friends cheated and tells them that they are an example of poor sportsmanship where Mondo disagrees, saying that Xever made him believe that it was everyone for themselves outside the streets, that the reason they won was because they looked out for each other and that friends don't let them fall off buildings. Xever calls Jason a traitor, where Mondo proclaims that his name is now Mondo Gecko and tries to punch Fishface, where Xever easily defeated Mondo and taunted him. Until Leo and the other Turtles arrive to the rescue. After Xever's defeat Mikey tells Mondo that Mr. X isn't his boss anymore and that Mondo is now free, happy that he is now free and Casey commenting that Mondo is actually an okay guy. The three cement their friendship by exchanging catchphrases.
In Annihilation Earth!, Jason and Muckman are revealed to have joined the Mighty Mutanimals, who return to aid the Turtles and the rest of their allies in stopping the Triceratons' plan to destroy the earth using the Heart of Darkness to create a black hole. During the fight Jason is easily knocked out after he attempted to attack a Triceraton. After the heart of darkness activates activates, Mondo, along with the rest of the earth, is sucked into the black hole.
In Earth's Last Stand, he was surprised to see the Turtles, April and Casey from the future participate n the battle and witnessed Fugitoid sacrifice himself to destroy the Black Hole Generator and the entire Triceraton fleet. He then saw the Fugitoid of the past take off with the past turtles, Casey and April and walked off with his allies.
- Robbie Rist, the voice of Mondo Gecko, also voiced Michelangelo in the 90's live action movies.
- Mondo Gecko's catchphrase "Cowabunga" is the turtles main battle cry in most versions of the series.
- Mondo Gecko was first seen in action during the Season 3 second opening sequence, which first appeared in "Return to New York".
- This version of Mondo Gecko started out as a human likes his Archie Comics counterpart.
- Mondo's T-shirt says "tokay", which is a type of gecko native to Southeast Asia. However, Mondo and his pet gecko Lars are leopard geckos, which are native to the western Indian Subcontinent.
- Nevertheless, Mondo being a mutant leopard gecko is inconsistent with some of his traits as a character. Mondo can cling to vertical surfaces, while leopard geckos are among the few geckos that actually lack this ability. And Mondo has no eyelids and must lick his eyes to clean them, while leopard geckos are among the few geckos who possess true eyelids. However, Mondo's counterpart in Amazing Adventures does have eyelids with a full range of expression.
- Jason's human hair resembles Mondo Gecko's in the Archie comics.