Sir Lionel was the younger son of King Bors of Gannes (probably Vannes in Brittany) and Queen Evaine. He was named from a birthmark shaped like a lion. After his father's death at the hands of King Claudas, Lionel and his brother, Bors the Younger, lived as captives at the usurper's court under the tutelage of their father's old friend, Sir Pharien. They were eventually rescued by the High-Priestess, Seraide, who turned them into greyhounds and whisked them away to complete their education with their cousin, Sir Lancelot, at the home of the Lady of the Lake.

Lionel and Bors accompanied Lancelot to Britain, where they became Knights of King Arthur's Round Table. One of his first tasks was to act, along with Bors, Gawain and Bedivere, as ambassador to the Emperor Lucius during the Roman Wars. Upon his return, Lionel accepted Lancelot's invitation to join him in adventure seeking. However, while Lancelot slept, Lionel tried to help three knights fleeing from a single adversary. All four were defeated by their opponent - a knight named Turquine - and thrown naked into prison. Lionel had to wait some time for release, since his companion was similarly imprisoned by the enchantress, Morgan Le Fay.

At other times, Lionel helped in the search for Sir Tristram; and then for Sir Lancelot, when he ran from the court in a fit of madness, having argued with Queen Guinevere over his extra curricula activities. During the Grail Quest, poor Lionel was captured by two evil knights who stripped him naked, bound him and foot and dragged him along the highway while thrashing him with thorns. His brother, Sir Bors, saw encountered them, but was obliged to abandon any rescue in order to save a nearby virgin from being raped. Unsurprisingly, Lionel was not best pleased at his brother's actions and, when next the two met, he threatened to run Bors through with his sword. Bors refused to defend himself, until a hermit and then Sir Colgrevance, were slain trying to break the two up. When he finally reached for his sword, a ball of fire descended from Heaven to stop the fight.

Sir Lionel, of course, sided with Lancelot when the schism with King Arthur opened up. He went with his cousin into exile and was crowned King of Gaul. He, however, returned to Britain to put down the last remnants of Sir Mordred's rebellion and was slain by the latter's son, Melehan, with fifteen other knights while travelling from Dover to London in search of Sir Lancelot.