Going back to his original black and white beginnings in the 6th issue of the 1987 Mirage Studio's comic Tales of the Teenage Mutant Ninja Turtles, Leatherhead was originally an exotic pet who got flushed down to the sewer, somehow ending up in an Utrom base. After being exposed to the same mutagen that would eventually change the Turtles, Leatherhead became a massive, humanoid croc. With intelligence mirroring that of Donatello's, he lived with the Utroms whom he considered his family. He was accidentally left behind during the Shredder's attack, which forced the Utroms to flee Earth. In his desperation to return to his family, he befriended Baxter Stockman and helped him create a new body while working on a transmat device so he could rejoin the Utroms. Leatherhead eventually met and befriended Michelangelo who found him living in the Turtles' old lair. After a battle between the Turtles and Stockman, Leatherhead learned that Stockman worked for the Shredder and attacked him. The resulting battle caused a cave-in, and despite Michelangelo's pleas for him to follow them, Leatherhead seemingly gave his life for his new friends.
Leatherhead later reappeared, surviving because of his tough skin, as a prisoner and object of experimentation in the lab of Agent Bishop. After being freed by the Turtles, he helped his old friends escape, and for a time lived in the Turtles' lair. However, Leatherhead's animal rages, already dangerous, were made more so by Bishop's experimentation. While in blind animal rage, Leatherhead struck Michelangelo, and woke up to believe that he had killed his friend.
Leaving the Turtles, a depressed Leatherhead found himself hunted by an extreme game hunter, Mr. Marlin, through the sewers. Luckily, he was saved by his friends who refused to give up on him and was delighted to discover Michelangelo was alive and well. In the end, he chose to live away from his friends due to his affliction, living in a nearby chamber (an abandoned subway station resembling the Turtles' lair from the second and third TMNT movies) so he would always be nearby, but still able to ensure their safety.
Leatherhead appeared several more times to help the Turtles, primarily in their battles with the Foot and Agent Bishop. He even joined them in attacking the Shredder's secret launch pad, and attacked his family's enemy with unmatched fury. However, the Shredder's minion Hun got in the way, and the two fell into the silo. Both survived, and Leatherhead returned to his lair. He is later revealed to have helped Donatello create the Monster Hunter gear used by the Turtles against Bishop's mutant creatures. He is also shown to have formed a deep friendship with Don, and is deeply saddened when his friend is mutated and he cannot cure him.
Leatherhead worked together with the other Turtles to capture Donatello. Soon after, he helped them penetrate Bishop's headquarters at Area 51. There, he was forced to fight the mutated Donatello. Reminded of the trauma he had suffered, he was tempted to exact revenge on Bishop but managed to control his anger with the help of his friends. He then went on to prove his brilliance by using Bishop's resources to devise a cure for Bishop's outbreak, despite his personal grudge against the man.
Leatherhead appears in the Season 7 episode "Wedding Bells and Bytes" as a guest at Casey and April's wedding at Casey's grandmother's farm. He remarks to Angel that he always cries at weddings. During the Foot attack, he helps save the other guests as the barn collapses and then helps fight the Foot.
We spend all our lives building walls, in order to keep us safe. But the walls do not work when the enemy is within. When you are defenseless, the enemy shows himself and he is a monster. It is our fate that the thing we fear the most is the thing we carry deep within ourself.
- In the episode What a Croc! This episode is partially inspired by Leatherheads' second comic appearance in Teenage Mutant Ninja Turtles #45.
- In the episode What a Croc! This is Leatherhead's first full appearance in the 2003 cartoon. He appeared as a shadowy figure in Secret Origins, Part 3.
- In the episode What a Croc! The title of this episode is a play on words in that it sounds exactly like the vulgar phrase "What a crock," which can be spoken to imply that something is false or nonsense.