Like his brothers, Leonardo was a simple pet shop turtle until exposure to toxic “ooze” mutated him. He was raised by Splinter and trained in ninjitsu along with his brothers, Donatello, Michelangelo, and Raphael.
In the Mirage comics, Leonardo is shown to be the eldest and most disciplined and skilled of the Turtles. He is the unofficial leader of the group, though he does cede authority to others, particularly Donatello, in situations where his brothers expertise are more suited.
In the first battle with the Shredder it is Leonardo who managed to best him by plunging his katana through the Shredder's torso. He offers his beaten foe his katana and says that the Shredder can regain his honor if he takes his own life.
Soon afterward, Splinter is possibly captured, and all of the turtles pursue to find their sensei. They are soon transported to the Triceraton homeworld where they must fight for their lives in a death arena. Eventually, they discover that the Utroms are not a threat and have, in fact, been rejuvenating Master Splinter.
Almost a year later, on Christmas eve Leonardo's training run was interrupted when he's attacked by an army of Foot Ninja. A fierce battle ensues on the rooftops of Manhattan with Leonardo holding his own against his enemies until he is shocked to see a familiar robed opponent. Surrounded by ninja he is severely beaten and dropped through April O'Neil's apartment window.
During his family's exile in Northampton Leonardo's defeat weighs heavily on him and he becomes consumed with self doubt. He would continually go out into the woods to hunt but come back empty handed. One day when returning from hunting he sees April falling through the ice of the lake she was walking on, he quickly uses his sword and cloak to make a lifeline for himself and pulls his her out. Splinter takes his four sons to the woods for intense training to rid them of the demons they picked up due to their defeat. The end of the story strongly suggests that Leonardo is himself once again.
Return to New York
A year later, in the story " Return to New York" a conflict erupts between Leonardo and Raphael when his brother demands they return to New York to confront the Foot with Raphael calling him a coward. The fight becomes physical with Leonardo plunging his wooden practice sword into his brothers chest. Eventually Raphael gains the upper hand and throws Leonardo through the wall of the barn before storming off alone.
Eventually returning to the city with Michelangelo and Donatello, he follows Raphael's plan to use the Triceraton Zog to invade the Foot head quarters. The plan goes well with the turtles and Zog besting an army of Foot but when faced with the
monstrously deformed Shredder Elite Raphael decides to ignore Leonardo's advice to stay together and goes off to confront the Shredder alone. Leonardo follows and rescues his brother from being killed after he is ganged up on by Shredder's Elite Guard. After accosting his brother Raphael says he'll go help their brothers and allows Leonardo to go after the Shredder.
Confronting the Shredder, he asks his enemy how he could still be alive, the Shredder tells him how the Foot used worms to bring him back to life. The two of them fight, with the fierce battle spilling out onto the roof. As the building begins to crumble both lunge at each other with Leonardo decapitating Shredder in mid air as the building explodes. After the battle Leonardo carries the corpse of their enemy back to the sewer lair. Later the turtles cremate the body to both honor their foe and make sure he could never return.
City at War
During the storyline "City at War" the turtles once again return to the city to investigate a rise in violence in the city involving ninjas. Making a new lair in a hollowed out water tower, the purpose was to once again confront the Foot. However, when looking over crime scenes, Leonardo begins to suspect there's more going on than they realize.
After they notice the Foot battling huge robots on the streets in broad daylight, the turtles intervene. Leonardo notices that the robots also belong to the Foot and the ninjas are actually fighting each other. The fight soon escalates as Foot from roof tops start firing guns and rocket launchers at each other and police helicopters. Shocked at the level of violence in which many innocent bystanders are killed he orders his brothers to retreat.
After the battle in the streets Leonardo starts to wonder how they could possibly combat the Foot during their conflict and even questions the validity of their war against the Foot Clan stating that it's time for them grow up. When the turtles point out that the war is for Splinter and for honor Leonardo laments that he can no longer live his life for his Master and that he's beginning to think that there's no place for honor in the world. During a brief encounter with one of the Shredder Elite, the ninja states that it's their duty to kill Leonardo as revenge for slaying their master. The Shredder Elite then commits seppuku realizing his mission has failed.
Weeks later, their new lair is discovered by a faction of the Foot led by a female ninja, the Foot use tazers on Leonardo to capture him, when the Foot become too overwhelming the rest of the turtles grudgingly retreat leaving their brother behind. When Leonardo wakes the female ninja introduces herself as Karai, the leader of the Foot in Japan who has come to restore order to the New York Foot in the wake of the civil war. Believing that the Shredder Elite are the main obstacles in attaining this goal she makes the turtles an offer, if they help her kill the Elite she'll declare a truce between the Foot and the turtles.
Leonardo becomes the main advocate for accepting Karai's offer believing they could finally live their own lives without the threat of the Foot Clan constantly hanging over them. Accepting the offer, Karai lures the Shredder Elite to the ruins of the Second Time Round Shop were the battle is fought. Perhaps some of the most escalated violence in their lives, the turtles fight with Karai against the Elite. Leo faces the most talented Elite for most of the battle with each resourcing to desperate tactics. Leonardo killed the last Elite by snapping his neck during an excruciatingly painful moment. In the end, Leonardo assures Karai that she owes them nothing but her word. When she tells them that they have a new beginning and to use it well, Leonardo responds "we will, Karai...we will".
Post City at War
Leonardo, along with Raphael, Michelangelo, Casey and April decides to stay in New York, however his two brothers opt to live in their own spaces. Leonardo laments that the family is breaking but soon finds his own place in a new sewer lair.
On the rooftops of Manhattan he is attacked by a Ninja who strikes him with Tetsu-Bishi with a drug which causes him to go blind. The Ninja in question claims to be an ex member of the Foot which the turtle had blinded in battle, while the Foot kept him on as a weapons polisher he mastered how to fight without sight so he could have revenge. The two do battle with Leonardo relying on his other senses to defend himself. The Ninja grows angry when he feels the turtle is taunting him and so rushes at him, protecting himself Leonardo plunges the sword into the Ninja's chest but when he feels the man die he realizes that the person he killed was a homeless person. Left alone, Leonardo realizes that the Ninja has exacted his revenge by getting him to kill an innocent.
After recovering from the poisoning, Leonardo finds himself training frequently. This often includes sparring matches with Karai. After a secret warrior targets the New York Foot Branch, Karai recruits Leonardo to help her solve the mystery of the warrior. Karai also unveils that these warriors have wiped out all other Foot branches, causing New York to be the last one. Karai unveils to Leo that the warrior has no confirmed gender despite looking like a man. The warrior also wields a Maquahuitl sword similar to that of the Aztec culture. Eventually, Leonardo faces another warrior of the same type, and he survives a killing stroke after a hard battle by flipping onto his stomach causing his shell to take most of the blow. This causes the sword to get stuck in his shell allowing enough time for the foot soldiers to shock the mysterious warrior with tazers.
Leonardo's investigation into the Maquahuitl-bearing warrior causes him to unearth documents about a secret tome that the Foot are searching for. Leo accosts Karai about not divulging to him the full truth, and Karai does not unveil any more new information. Later, Leo discovers that Master Splinter had the tome all along, and he uses incantations in it to create a doorway to the Battle Nexus. While there, he meets Oroku Yoshi, The Shredder from a different universe.
The Distant Future
Not many stories have been told of Leonardo in the future. He is often depicted without his eye bandanna and with a single, electronic, light blue katana.
At one point, summoned by a god to prevent its aging, four Leonardos meet to complete a single mission. The four Leonardos represent him at different time periods including one somewhere between the ages of 80 and 120. This is the only version of Leonardo that remembers the events after a mindswipe.
Leonardo lives on to become life partners with Radical. However, Radical and Complete Carnage's constant struggle shown previously comes to an end when Complete Carnage kills Radical in front of Leonardo. Leonardo then walks the Earth - after his love has been murdered - meditating and learning further mystical arts. Finally, he confronts Complete Carnage in Tokyo in order to forgive him for Radical's death. This, however, does not go to plan due to Complete Carnage holding multiple women hostage. In a moment of weakness, Leonardo's anger erupts and he kills Complete Carnage. This is his final kill.
In August 2011, IDW launched a new ongoing series Teenage Mutant Ninja Turtles co-written by creator Kevin Eastman and Tom Waltz with Dan Duncan supplying the art.
Similar to his Origin in the Mirage Universe, Leonardo was exposed to a chemical compound that caused him and his brothers to mutate drastically into humanoid figures. Leonardo continues to take on the role of methodical trainer and leader of the team.
Leonardo is considered one of the most accomplished swordsman in the Turtles' Universe. The only combatant near to his skills may be Karai.
Leonardo often fights with two katanas but is just as skilled with one. Leonardo's skills stretch into other weaponry, but, at these moments, it is often as a last resort or for training purposes. One notable instance is when he fought Cha Ocho with a pocket knife, winning easily.
Leo has also shown a knack for the mystical arts in his older age such as when he followed an ancient manuscript of Splinter's to discover the Battle Nexus. He is often shown with a mastery over meditation; gaining the ability to float in his older age.