Leonardo was a pet turtle along with three others who had been bought by a boy, but the boy tripped and the four turtles fell into a sewer drain. At the time, Hamato Yoshi had been hiding in the sewer from his enemy, Oroku Saki, and had become a friend of the rats living down there. Hamato was overjoyed to find four new friends, and welcomed the turtles into his home. At some point after, Hamato noticed the turtles covered in a glowing substance, and he touched the substance himself. The turtles had most recently touched Yoshi, causing them to quickly evolve into humanlike turtles. Hamato, having just been exposed to the rats, became a human rat. Hamato saw the turtles' potential to challenge the one responsible for the mutations to change him back to a human, and trained them in the art of ninjutsu, and naming them after some of his favorite Renaissance artists – Michelangelo, Leonardo, Raphael, and Donatello.
Some years after, April O'Neil was giving a news report on a recent break-in at a science lab, and a scientist used witnesses of the thieves and bits of rope to conclude the thieves were most likely ninja. April and her crew went to the next potential science lab robbery, only to moments later be chased by a group of street gang members. April managed to slip into a sewer drain, but the gang members followed. Suddenly, a second group of ninjas appeared and took out the gang. April fainted when she saw Leonardo and the other ninja were turtles. Leonardo and the others took her to their Master Splinter, (Hamato Yoshi's nickname given by the Turtles) who explained the Turtles' origin to her. April, however, believed that the Turtles were the thieving ninja, but the Turtles made a deal: they would help her find the one behind the thefts and get her a story, while she would try her best to find the Mutagen creator. She took the Turtles to the surface and bought the Turtles coats and fedoras to keep the Turtles' bodies unrevealed, and went to the scene of the crime, finding only a matchbox with the words "Ninja Pizza" on it. They found and went into a restaurant by the name of Ninja Pizza and ate there.
April left and scouted the neighborhood to find a security building, and heard orders being given out to rob a science lab. April was going to phone Channel 6 (the news station she worked for) and tell them she had found the ninja, only to be captured. Shortly after this, Leonardo and the others expressed their concerns for April's long disappearance and found her belongings, finally finding her tied to a chair on top of a building. A group of ninja assaulted the Turtles, who found out their attackers are just robots. Oroku Saki, watching the fight from a closed camera system, demanded his remaining Foot Soldiers to set off the sprinkler system in the building. Leonardo, April, and the others managed to escape before the building exploded from the pressure.
In the 1987 TV series' theme song lyrics, Leonardo is said outright to be the leader of the TMNT, and there is little disputing this; his orders are usually followed, and he is a very serious do-gooder who hardly ever makes wisecracks. In one episode, he suffered a crisis of confidence and left the group to do some soul searching; his brothers all tried their hands at being the leader in his stead, only to find that none of them could match his leadership skills. He eventually realizes that his brothers needed him, and came back to them in time to help save the Earth.
He was attracted to a young kunoichi named Lotus Blossom, a swordswoman prodigy from Japan who was hired by Krang to replace Shredder, whom she easily defeated (along with Rocksteady and Bebop). She and Leonardo dueled to a standstill before she resorted to a trick sword to knock him out. When they met the second time, she tried to convince him to join her as "ninja for hire", but he refused. She turned on Krang and escaped to continue her mercenary lifestyle, telling Leonardo that there was little gold in goodness, though she hoped that they would one day be on the same side. She reappeared in Farewell, Lotus Blossom, where Leonardo seems to have formed a supportive, more trusting platonic rapport with her.
Leonardo and Raphael in this incarnation get along better than the other incarnations. There was one exception to this, namely in the episode Turtle Trek when Raphael challenged his leadership by saying to Leonardo they should get a new leader but Leonardo refused to fight him.
When the cartoon series starts out, he is shown with having a very level head, similar to his leadership qualities in the comic. However, by season 2 his voice got slightly higher and by season 3 he sometimes acted like a leader of a group of superheroes, which is different from the original, deeper pitch in the first season.
Leonardo also seems to enjoy reading. For example, many times when the Turtles are at home, Leo is reading a book. In the episode Four Musketurtles, he is the only Turtle that read The Three Musketeers. Another good example is in Leonardo is Missing; while the other Turtles go to an arcade, Leonardo stays at the lair and reads.
In the season 3 episode Take Me to Your Leader, Leonardo gives up his leadership and walks away after a dream convinces him that he is no longer a good leader. The others have to find him, and stop Shredder, Krang, Bebop and Rocksteady from draining energy from the Sun with a Solar Siphon and store it in solar batteries. However, Leonardo returns when he spots a bridge collapsing due to snow. After a man says that everybody talks about the weather but nobody does anything, Leonardo realizes his responsibility and begins to search for his brothers. He later finds them, and together they save the Earth.
In the Season 6 episode Snakes Alive, it is revealed that Leonardo has paralyzing ophidiophobia, fear of snakes, but confronted it. He later stated that he no longer is terrified of snakes, but still dislikes them.
In Season 10, he undergoes a Hyper Mutation while he and his brothers try to stabilize their unstable mutation. Leonardo arguably had the roughest time of it; even after a cure was found, he repeatedly hyper-mutated and went on violent rampages, and was only cured after being turned into a living bomb by Lord Dregg.
In the original English language version of the 1987 cartoon show, Leonardo's voice actor was Cam Clarke (who also voiced Rocksteady). Leonardo is generally considered to be Clarke's "breakout role" and is still one of his best known parts.
He, along with the rest of his team, make an appearance in Turtles Forever. However they were voiced by different actors.
He and the other members also make a few appearances in the 2012 series, first making brief cameos in "Wormquake!" and "The Wrath of Tiger Claw". Unlike in Turtles Forever, he and the other characters are voiced by their original voice actors. All four Turtles appear in "Trans-Dimensional Turtles", "Wanted: Bebop & Rocksteady", "The Foot Walks Again!", and "The Big Blow-Out!".
Skills, Powers and Abilities
- Leadership: Leonardo's bravery, determination, and willpower makes him a great leader. And due to his heroic and selfless nature, Leonardo is willing to put his own life on the line to keep his friends and family safe.
- Kenjutsu: Leonardo is a highly skilled martial artist and master swordsman. Leonardo was trained by Splinter in Japanese sword fighting. As his signature weapons are his twin katana, he is highly skilled in the sword fighting style of the Niten Ichi-ryū,(二天一, "two heavens as one"), making him an excellent swordsman. He makes use of his katana for skilled offense and defense, and he can slice clean through enemies. However, it is highly noticeable that as great as Leonardo's skills may be, it is nowhere near to the superior level of skills his 2012 counterpart has demonstrated. For instance, in Wanted: Bebop & Rocksteady, Leonardo has been trained by his 2012 counterpart in both armed and unarmed combat during his basic training.
- Shurikenjutsu: Leonardo can throw shuriken with remarkable accuracy.
- Expert of Ninjutsu: Leonardo is known as to be the best combat fighter in the team of Ninja Turtles. His prowess allows him to defeat and take down thugs and many foot-bots with ease.
- Enhanced Strength: Leonardo possesses very high physical strength, as a result of his mutation altering his physiology and his training with Splinter, Leonardo's strength has been amplified to enhanced levels, and thus he can easily overpower humans. Leonardo can push and lift things much bigger and heavier than himself, run at incredible speeds while weighted down by objects larger than him, as well as throwing people over his head, knocking out average criminals with a single hit, trade blows with the extremely strong Slash, knock back several foot-bots at once, and send criminals flying. Additionally, Leo was able to send Krang flying several meters across an entire roof with a single kick.
- Enhanced Speed: Despite being a turtle, Leonardo can run and move at speeds faster than an ordinary human. His speed allows him to free run on rooftops without losing momentum, and easily outrun normal humans. He also possesses great reaction speed, allowing him to dodge projectiles such as the Kraang's laser blast and enemy attacks. He can even move things at enhanced speeds too, like twirling his katana blades.
- Enhanced Endurance: Leonardo has several times demonstrated extreme resilience to damage. He can survive getting caught in the crossfire from all angles by Bebop and Rocksteady's blasts, withstand multiple hits from Foot-bots unscathed, being pummeled with swords, and smash through several robots with his fist without discomfort, and take take hard hits from Shredder. Leonardo's toughened flesh also allows him to endure collisions that would overall pulverize ordinary beings, as he can survive Rocksteady charging at him at full speed (which only knocked him out cold briefly). Additionally, Leonardo can survive impacts from several stories heights, only to shake it off when landing.
- Enhanced Agility: Leonardo's agility, dexterity, balance and bodily coordination are enhanced. His bones, muscles and joints are twice as elastic than the average human which, combined with his great strength, allows him to perform some astonishing acrobatic maneuvers with no difficulty whatsoever, as well as swiftly outmaneuvering multiple foot-bots effortlessly.
- Enhanced Stamina: Leonardo possesses exceptional stamina, which allows him to exert himself at his peak performance for an hour without stopping or slowing down, as well as running massive distances over rooftops without stopping.
- Enhanced Healing: Leonardo can heal and regenerate from harm much faster than the average person. After enduring multiple hits from Bebop and Rocksteady, he was still able to keep on fighting. He rapidly healed from multiple hits from Bebop and Rocksteady and regain consciousness really fast.
- Retractable limbs into Shell: Though rarely (if ever) depicted, he has retractable limbs and the normal ability of a turtle to retract his head and limbs into the safety of his shell.
- Stealth: Thanks to his ninjutsu training, Leonardo is able hide in the shadows and move through them without being seen or detected.
- Skilled Strategist & Tactician: Leonardo is a skilled tactician, as he has a good line at forming strategies.
- He is allergic to ragweed, as mentioned in Unidentified Flying Leonardo.
- He enjoys video games, and seems to be quite skilled at them.
- He suffered from a crippling fear of snakes, though he later overcame this phobia.