Some TMNT stuff was always meant for grownups.
Leonardo in the Mirage comics is the most serious of all versions. Leonardo is depicted as the oldest brother and the calmest of the four. In Volume 1 of the original comic series by Kevin Eastman and Peter Laird, Leonardo plays an important role. At the beginning of the series, he is fifteen. In the early stories, he is not explicitly referred to as leader, though he usually speaks for the team. Further in the series, he openly states his position as leader to Raphael in Teenage Mutant Ninja Turtles #44. He is the one to usually take charge of the Turtles when Master Splinter is not present. He is often at odds with his more hot-headed brother, Raphael, as they are just too different. He is the team's primary strategist (except in situations related to technology - Donatello's field). Leonardo prefers considering a plan before he attacks. Raphael, however, would rather attack the enemy directly. This was reflected, for example, when Splinter was gone. Leonardo tried to keep a cool head, but Raphael simply ran away and wanted to find Splinter on his own. But when it comes to the crunch, the brothers can always rely on each other.
Of the four brothers, Leo trained the hardest and he always tries to internalize Splinter's teachings. This diligence, he also tried to transfer to his brothers, but he rarely succeeded. In addition, Leonardo draws from a very highly developed sense of honor, and when other characters share his sentiments, at least in approach (such as Karai and Miyamoto Usagi), he becomes quite close to them.
In Leonardo #1, Leonardo goes out for a run on the rooftops of New York City and is ambushed by the Foot Clan. He puts up an admirable fight against the army of Foot Ninja, but is eventually overwhelmed by them. Rendered seriously injured and nearly unconscious, he is thrown through April O'Neil's apartment window. The remaining Turtles and Splinter are forced to continue the fight, but even with the aid of Casey Jones, the odds are against them. In the end, the building catches fire and the police arrive, but they secretly escape to Northampton.
In Teenage Mutant Ninja Turtles (vol. 1) #11, when the four Turtles, Splinter, April O'Neil and Casey Jones retreat to a farmhouse in Northampton, Leonardo recovers physically from his wounds. However, he seems despondent and lacking a great deal of confidence. He repeatedly attempts to hunt for deer, but he is unsuccessful. While out hunting, he sees April fall through ice into a lake, and he rescues her. In subsequent issues, it is implied that Leonardo has regained most of his confidence.
In the storyline, "Return to New York" (Teenage Mutant Ninja Turtles (vol. 1) #19-21), Raphael demands that the Turtles return to New York to confront the Foot Clan and the Shredder, accusing Leonardo of cowardice. The two brothers argue and soon come to blows. Leonardo is beaten by Raphael, who throws Leonardo through the wall of the barn and leaves alone. Along with Donatello and Michelangelo, Leonardo returns to New York shortly thereafter and reunites with his wayward brother in the old sewer lair. The three go along with Raphael's plan to storm the Foot Headquarters. The four Turtles infiltrate the Foot Clan headquarters, where once again Raphael goes off on his own to confront the Shredder, only to be ambushed and beaten by Shredder's Elite Guard, but is rescued by Leonardo. This prompts Raphael to finally cede to Leonardo's leadership out of respect to his older brother and returns to aid Michelangelo and Donatello, leaving Leonardo to fight the Shredder. Leonardo engages in a bloody battle that spills out onto the rooftop of the building. Leonardo ends the battle by decapitating the Shredder just as the building implodes. The Turtles later burn the Shredder's corpse in a funeral pyre in a nearby Manhattan harbor.
In the "City at War" storyline (Teenage Mutant Ninja Turtles (vol. 1) #50-62), a feud between various factions of the Foot Clan over leadership breaks out. Fighting ensues in the streets of New York, with the Turtles and the civilian populace caught in the middle. In the story, Leonardo grows weary of constant battle with the Foot Clan and seems fraught with indecision. The Turtles are approached by Karai, the leader of the Foot Clan in Japan, who has come to New York to unite the Foot. She presents the Turtles with an offer of a truce between the Foot and the Turtles if they help her kill the Shredder's Elite Guards, which are a major obstacle to her reorganizing the Foot. Leonardo persuades his brothers to accept Karai's offer (much to Raphael's chagrin) and all four Turtles work with Karai to eliminate the Elite Guard.
In Volume 2 of the Mirage Studios comic, the Turtles begin living in separate places. Leonardo decides to live in a newfound sewer lair. Michelangelo and Raphael notice a change in Leonardo and note that he seems more easygoing, though Raphael points out that his and Leonardo's natural order is to be "buttin' heads."
In Volume 4, Leonardo still leads his brothers (all four now in their thirties) in fights against crime. The conflict between Leonardo and Raphael seems to be greatly lessened as both characters have "mellowed," being in their thirties. When the Utroms make a very public arrival on Earth and reveal alien life to humans, however, the Turtles become free to mingle in everyday society. The Turtles also help the Utroms acclimate to life on Earth and work alongside the Foot Clan as security. Leonardo has a new rival in one Foot Clan member, Cha Ocho, which stems from an encounter the two had years earlier. Karai approaches Leonardo for help when a mysterious force begins attacking various Foot Clans; only the New York branch is left intact. His investigation takes him to the Battle Nexus, where he meets Oroku Yoshi (who wears armor similar to the Shredder's).
In the future, an aged Leonardo has resigned from killing, after his lover Radical was murdered by Complete Carnage. His revenge killing on Complete Carnage was the final time he would take another life.
This Leonardo had a brief appearance in Trans-Dimensional Turtles.