As a Knight of the Round Table, Lancelot travels with King Arthur and Sir Percival to Excalibur, where Arthur pulls the legendary sword from the stone and becomes the ruler of Camelot. Later, Lancelot leaves Camelot and becomes a "sword-for-hire", employed by King George as his general. Lancelot eventually captures Snow White and brings her to King George, who gives Snow some water to drink. However, the water is cursed and leaves Snow unable to bear children. Even Lancelot is shocked by the king's actions, and later decides to help Snow. Snow, Lancelot and David go to Lake Nostos, in which the water from the lake has the ability to restore anything. Snow drinks what is left of the water from the lake, and is now able to produce a child with David, which they do. When the first Dark Curse is unleashed by Regina Mills, a small portion of the Enchanted Forest is shielded and saved by Regina's mother, Cora, which means that Lancelot and the people who are protected by Cora's shield are not sent to Storybrooke.
Cora apparently kills Lancelot and starts masquerading as him, until she is caught out by Snow - who has returned to the Enchanted Forest with her daughter, Emma - causing Cora to reveal to them that she killed Lancelot a long time ago, and pretended to be him so that the people in the safe haven would do what "Lancelot" told them to do. After uncovering Cora's ploy, the heroes tell everyone that Lancelot died valiantly.
Lancelot's former allies King Arthur and his knights greet Emma, who has become the Dark One, as well as Snow, David, Regina, Hook and Henry, and take them to Camelot. However, Lancelot reveals himself to Snow, and warns her that Arthur cannot be trusted. However, King Arthur's forces capture Lancelot and throw him into the dungeon, where he meets Merida. They are later freed by Merlin, and Lancelot returns to his mother.
- Lady of the Lake (mother)
- He is based on Sir Lancelot from Arthurian Legend.
- Lancelot's nickname, the Leviathan, could be a reference to Lancelot's mother, the Lady of the Lake. Lancelot also claims that he was raised near a lake. He was also probably named after the Leviathan.
- S2, E03: "Lady of the Lake" (flashback)
- S5, E01: "The Dark Swan" (flashback)
- S5, E03: "Siege Perilous" (flashback)
- S5, E04: "The Broken Kingdom" (flashback)
- S5, E06: "The Bear and the Bow" (flashback)
- S5, E10: "Broken Heart" (flashback)
- S6, E07: "Heartless" (voice)