Mondo Gecko began his life as a normal gecko who fell into New York City sewers when the TMNT were mutated by the ooze, and he also became mutated. However, Mondo Gecko was picked up and carried away by Mr. X. One night, Michelangelo had a bad dream and woke up. When he heard noise up on the streets, he went up and met robbers. Among the robbers were Mondo Gecko, now a mutated and talking lizard who was raised by Mr. X. Mondo Gecko was now a member of a gang who worked for "Mr. X", the man who took Mondo away. Michelangelo convinced Mondo Gecko to turn on Mr. X. Together they fought Mr. X. After this, Mondo Gecko ends his career as a criminal and moves to the sewers and becomes "neighbors" with the TMNT and Splinter.
Mondo would return after being enlisted by the turtles to let himself get captured by Dirk Savage, a mutant hunter who had captured the likes of Tokka, Rahzar and the Punk Frogs, in order to find out where all the mutants were being taken to.
- Unlike other incarnations where he was regularly called "Mondo" as if Gecko were a surname or species identifier, other than introducing himself as "Mondo Gecko", he was regularly called "Gecko" by both allies and enemies, to the point where even when Mr. X was teaching him graffiti in a flashback, he began writing "Gecko".
- Character gallery: Mondo Gecko
- Appearances of Mondo Gecko