In the original Mirage comics, Leatherhead presumably started life as an exotic pet which ended up in the sewers after being flushed down a toilet. He was found by two Utrom scientists and taken back to their lab at the TCRI facility for study. While there he was accidentally exposed to a mutagen and grew larger and more intelligent. The aliens decided to educate the creature and allowed him to live with them.
Despite acceptance by the Utroms, Leatherhead still had a strong connection to the sewers and would go exploring the underground tunnels for days before returning to his alien friends. One day however he returned to find the TCRI facility destroyed. Assuming humans were responsible and hoping that the Utroms had teleported back to their homeworld, he gathered what little technology was left and retreated into the sewers, working on a device to contact the Utroms.
Encountering the Teenage Mutant Ninja Turtles in the sewers while they were being attacked by a big game hunter called Marlin who was ironically searching for a giant alligator that had been sighted in the sewers. The Turtles and Leatherhead realized their connection and after defeating Marlin the Turtles allowed Leatherhead to live in their original lair which they had since abandoned so he could work on his device in seclusion.
Years later the lair is found by a group of Foot Clan members. Leatherhead who now has an eye patch attacks the intruders by a lone Foot ninja convinces Leatherhead to accept them as allies. The mutant reveals that he's been working on a transmat to teleport him back to the Utrom home world but his over-sized hands make it difficult to do so but now that he had friends with smaller hands he can finally achieve his dream. The Turtles arrive at their old lair and a fight breaks out with the Foot only to be diffused by Leatherhead and both groups grudgingly work together to finish the transmat.
Once the device is complete the gator steps in but when activated he is not teleported away, but rather burned by energy. In a violent rage Leatherhead starts to lash out at Foot and Turtle alike swearing vengeance on both. Years later in Tales of the TMNT Vol 2 Leatherhead came across Raphael in the sewers and in a primal rage attacked him. Coming to his senses before he could do any permanent damage he explained that he was suffering from uncontrollable fits of rage where pure instinct would take over. More mysterious was the fact that his eye had healed, he'd grown much larger and would black out for days with no memory of what transpired.
He showed Raphael a completed transmat but explained he had no recollection of building it. After the arrival of the other Turtles, the machine activated and a group of Utroms with ray guns captured Leatherhead teleporting him away, and they left behind explosives which destroyed the Transmat behind them.
The story was followed up issues later revealing that the Utroms that had captured Leatherhead were part of a secret religious sect called the "Illuminated" who believed it was their god given duty to cleanse the universe of all violent species so that peaceful ones can live without fear or hindrance. They had been secretly experimenting on Leatherhead to make him the perfect weapon so now they could make an army of his clones to attack other worlds.
Thankfully Leatherhead was rescued by a group of Utroms led by an Utrom called Ribky and accompanied by the Turtles. After fighting their way out of the Illumitated's, Ribky stayed behind and sacrificed himself to destroy the base. The other Utroms explained that Ribky was in charge of waste disposal at the TCRI facilty and so was responsible for the accidents that mutated Leatherhead and the Turtles. Feeling responsible for his creations he had to make sure they were all okay before he died. In the end Leatherhead requested to be sent home to Earth.
In the present comics Leatherhead secretly attended Master Splinter's funeral from afar though he was noticed by the Turtles, who were surprised that he somehow knew of their master's death.
When Raphael was mutated into a monstrous creature he roamed the sewers and eventually battled and defeated Leatherhead. When Raphael's mind was clear and he could think again he stated that he hoped he hadn't hurt Leatherhead, suggesting the Turtles and the alligator were on good terms.
Leatherhead in Archie Adventure Series Comics
Jess Harley was turned into the Crocodile man Leatherhead by the swamp witch Mary Bone after he attempted to steal the turn stone. Shredder manipulated Leatherhead into fighting the TMNT, convincing him they were villains in league with Mary Bones.
After Leatherhead learned the Shredder was the true villain he went into a rage and nearly fell to his death, but was rescued by Cudley the Cowlick and taken to Stumps Asteroid to compete in the Intergalactic Wrestling Federation. Leatherhead remained in the IWF until the forces of Maligna were threatening to invade Earth, Leatherhead along with fellow IWF members Wingnut and Screwloose returned to Earth and joined forces with Jagwar, Man-Ray, Mondo Gecko and Dreadmon forming the Mighty Mutanimals.
The team fought numerous threats including the Four Horsemen alongside the TMNT, but soon after that battle Null found four new henchmen, the cyborgs called the Gang of Four, Leatherhead and the rest of the Mutanimals were ambushed and shot to death in their base by the Gang of Four.
Powers & Abilities
As a giant humanoid alligator, Leatherhead has super human strength. His scaled hide proves to be very durable. His ferocious nature makes him a deadly combatant. He also has fantastic senses, such as smell and hearing.
The 1987 Cartoon
In the famous classic cartoon, Leatherhead served as a foe to the turtles, in which he tried to enslave them then tried to get revenge on them. He lived in the Florida swamps and speaks with a Cajun accent. He was voiced by Jim Cummings & Peter Reneday.
In the current CGI Teenage Mutant Ninja Turtles animated series, Leatherhead is featured in the more comic-book accurate role as an ally of the TMNT, he is revealed to have been born by the exact mutagen that created the TMNT, but rather then being raised by Splinter, was raised by the Utrom. He helped fight the forces of the rogue Utrom, Krang during numerous occasions in the series.