Leon was an ancestor to both Altaïr Ibn-La'Ahad and Ezio Auditore da Firenze. He was not an ancestor of Ratonhnhaké:ton. Leon learned some Italian as a result of some of his ancestors being Italian.
- "I think think you are disturbing Erika's lovely day."
- ―Leon before facing Jackson
Leon was born to Jane and David Brown in 1996. He was trained by his father, unknown to Leon, to make him an Assassin starting at age nine. Leon learned combat, climbing up buildings, some climbing up trees, and improved on his swimming. In high school, at age 16, he was at a prom night with his friend Frank. Frank asked him if he liked any of the girls he saw, and he picked out one named Erika. When he tried to talk to her though, first she ignored him, and then when she actually listened, she had to go. Then he decided to try to talk to her in private at her home, but when he got there, his nemesis, Jackson Trent, was there to hurt Erika because he hated her. Leon arrived and fought Jackson, using Jackson's anger to his advantage, because he had been taught to keep a cool head in combat. When Leon defeated Jackson, Jackson ran from the scene. Erika thanked Leon, and they fell in love. At age 17, Leon demanded to know what secrets were being kept from him from his father. David was about to tell Leon, but then Jane ran in with many System Corporation soldiers behind her. (System Corp. is a branch of Templars split off from Abstergo after Vidic's death.) The soldiers shot Leon's parents to death right in front of him. After that, Leon became a cop.
- "What the hell happened here?"
- ―Leon during the Mansion Incident
As a cop, Leon's first assignment was going to a mansion where System Corp. was, because they were doing something suspicious. When they got there, they encountered zombie-like beings that were infected by a System Corp. virus. In the initial attack, Jenkins, another operative, was killed, and Leon was separated from the group. Leon fought his way through solo, or so he thought. There was really an Assassin there named Altaïr, after the original one. He saved Leon many times, because the truth was that Leon was an Assassin. Leon encountered a System Corp. leader named Frederic there. They battled, but Frederic escaped. When Leon managed to find Frank, who was a cop too, he was wounded, and the last except Leon left. When they got to the exit, Frederic shot Frank and got away. When Leon got out, he saw the whole town that hadn't escaped infected. His mission had been a diversion so that there were few people left to protect the town. Leon fought to try to find System Corp's facility. There he finally met the other Assassin, Altaïr. They got to the facility together, where Altaïr told Leon that he was an Assassin. They fought their way through, but they encountered Frederic again, who revealed that he was Altaïr's father and Leon's uncle, making the two Assassins cousins. Frederic killed Altaïr, and then was killed by Leon. Leon managed to find a bomb to stop the infection with the guidance of the original Altaïr's spirit. Leon then came to the Assassins.
- "I will follow in the footsteps of my ancestor Altaïr."
- ―Leon planning his initiation
Leon was put into the Animus to find his ancestry. The Assassins found that he was an ancestor of Altaïr Ibn-La'Ahad by seeing Altaïr's initiation, where he cut off his finger. When they pulled him out, they told him he wouldn't have to cut his finger off, but he would be initiated. Leon, however, decided to have his ring finger cut off anyway, to follow Altaïr's example. Before that, he had to assassinate Alan Rikkin. After his initiation, Leon was sent on many missions against Abstergo, now that System Corp. was gone. Leon found out that his old nemesis, Jackson, had been a Templar all along, and was sent on a mission to kill him in New York. Leon performed the dangerous stunt of air assassinating Jackson from the top of the Empire State Building. He came into contact with Erika again later, and they got married at age 27. Leon continued his Assassin work for a while. At age 35, he had two children with Erika, named Jason and Jared. At age 45, Erika was killed by a Templar attacking Leon. Leon, in his rage, stabbed the Templar violently, and for the first time, losing his cool in a fight. This made him reckless, and if it weren't for the help of his fellow Assassin, Drayden, he would have been killed as a result. Just before becoming Mentor, Leon acquired an Apple of Eden. At age 50, Leon became the Mentor of the Assassins. As Mentor, he did not go out into the field quite as much. Thirty-seven years later, Leon died of a heart attack. Before he died, he was able to communicate with his ancestor Taron, who accessed his memories through the Animus. He told Taron to make sure that the Assassins remembered Leon through him, and that he had lived a good life, and that its end was drawing near. He said that if he could go back, he wouldn't change a thing, because all the bad things did not last, as they led to good in the end. He then sat in his seat, and died in it peacefully.
Jackson: It's amazing...isn't it Leon. How we went from playground foes to sworn enemies on the field of battle.
Leon: I agree with you completely.
Jackson: (laughs, then coughs blood) And now it looks like you've won Leon. For the last time. (falls dead)
Leon: Farewell, my nemesis. Requiescat in pace.
- "My father...was a great man, and was loyal right up to the end."
- ―Jason Brown
Leon was remembered well by the Assassins afterward. Many statues of him were made, and he was remembered as a hero. His memories were accessed in the Animus about 160 years later by an Italian Assassin named Taron.