Going back to his original black and white beginnings in the 6th issue of the 1987 Mirage Studios 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 crocodile. With intelligence mirroring that of Donatello's, he lived with the Utroms whom he considered his family. He was accidentally left behind during the Utrom Shredder's attack, which forced the Utroms to flee Earth.
Leatherhead first made a shadowed cameo in Secret Origins, Part 3 escaping from the TCRI building during the raid. He was first seen in the episode What a Croc, where he was helping Baxter Stockman create a new body, while working on a transmat device so he could rejoin the Utroms and had built the Turtlebot. But Stockman only wanted to take advantage of him. Leatherhead eventually met and befriended Michelangelo who found him living in the Turtles' old lair. After a battle between the Turtles and Stockman, unfortunately for Stockman, the doctor accidentally allowed Leatherhead to know he once worked for the Shredder; since the Utroms had adopted and told him about the Shredder, Leatherhead immediately got angry and grabbed Stockman. Eventually Stockman bombed the location which caused a cave-in, and despite Michelangelo's pleas for him to follow them Leatherhead seemingly sacrificed himself to save the Turtles.
Later in season 3 it was revealed that Leatherhead had in fact survived the ordeal because of his tough skin, but had been taken captive by Agent Bishop and abused as a guinea pig for genetic experiments in his lab - an event that inflicted Leatherhead with severe emotional wounds. 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, with his enhanced aggression, made it impossible for him to live together permanently with the Turtles, 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 and help them with his intellect as much he could.
He reappeared in a number of subsequent episodes as an ally to the Turtles primarily in their battles with the Foot and Agent Bishop, including the raid on the Shredder's building. He even joined them in attacking the Shredder's secret launch pad, and attacked his family's enemy with unmatched fury. He faced the Shredder's minion Hun who got in his way, and during the battle the two fell into the silo. Both survived, and Leatherhead returned to his lair but lived by himself to avoid accidentally hurting the Turtles due to his anger issues that furthered increased from Bishop's experiments on him. 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.
During the fourth season, Leatherhead played an important role in working together with Don in containing the mutation of local creatures from the mutagens that were created by Baxter Stockman previously during his faked alien invasion. However, when Donatello is accidentally exposed to the mutagen, Leatherhead is deeply saddened he cannot cure him and works with the other turtles in capturing the mutated Don. Leatherhead later discovered that the combination of the mutation from Bishop's outbreak was mixing with the original Utrom mutagen in an unstable reaction, it would kill Don unless if it was cured.
Leatherhead worked with the Turtles and Splinter in breaking into Area 51 where Bishop headquarters was. While Bishop sent the three turtles to steal the Heart of Tengu from Karai, he was forced to fight the mutated Donatello. Reminded of the trauma he had suffered, he was tempted to extract revenge on Bishop but managed to control his anger with the help of his friends. Despite his personal grudge against Bishop, Leatherhead was able to use the details that Bishop and Stockman had on the new mutagen, and using Bishop's resources, he is able to cure Don, and the other animals and people mutated from the outbreak.
Back to the SewerLeatherhead can be seen in Donatello's memory in the episode . Leatherhead appears in attendance for Casey and April's wedding at Casey's grandmother's farm in . He is spotted crying, and remarks to Angel that he always cries at weddings. During the Foot attack, Leatherhead helps save the other guests as the barn collapses and then helps fight the Foot.
Leatherhead is a combination of a docile scientist and inventor with the savage ferocity of a crocodile. This combination makes him dangerous but valuable to the turtles. Before Agent Bishop made his aggression worse, he could control it to a certain extent.
- "Secret Origins, Part 3"
- "What a Croc!"
- "Worlds Collide, Part 2
- "Worlds Collide, Part 3
- "Touch and Go"
- "Bishop's Gambit"
- "Exodus, Part 1"
- "Adventures in Turtle Sitting"
- "Good Genes, Part 1"
- "Good Genes, Part 2 "
- "Identity Crisis" (Cameo)
- "Wedding Bells and Bytes"