Pellinore first appears in the Post-Vulgate prose cycle and in Thomas Malory's Le Morte d'Arthur, where he is introduced as the King of Listenoise, a realm that some identify with the Lake District in northern England and others with the Isle of Anglesey. He is a knight of such prowess that Merlin himself declares him one of the best in the world, a man of great strength and terrible single-mindedness. His most famous trait is his obsession with the Questing Beast, also called the Beast Glatisant, a monstrous chimera with the body of a leopard, the head and neck of a serpent, the haunches of a lion, and the feet of a hart. The beast makes a sound like thirty couple hounds questing from its belly, and according to Merlin's prophecy, only Pellinore and his bloodline are destined to hunt it. Pellinore had pursued the creature for a year when he first met King Arthur, sleeping by a fountain near Carlion, and the encounter would change both their lives.

The meeting was not friendly. Arthur had seen the Questing Beast drinking from a pool after a disturbing dream that foretold the destruction of his realm by Mordred, the son he had unknowingly begotten on his half-sister Morgause. Pellinore, desperate and on foot after the beast had killed his horse, demanded Arthur's mount. When Arthur refused, Pellinore took it by force, and the two knights jousted three times. Pellinore was the victor, and in their final combat he broke Arthur's sword, the very blade that the young king had drawn from the stone to prove his right to rule. Pellinore dismounted to fight Arthur on foot, and the battle was so brutal that blood filled the field. When Pellinore had Arthur at his mercy, helmet removed and sword raised to strike off his head, Merlin intervened. The wizard cast an enchantment that put Pellinore to sleep for three hours, saving Arthur's life and his kingdom. When Pellinore woke, Merlin revealed Arthur's identity and prophesied that Pellinore would one day do the king great service, that he would have two sons named Percival and Lamorak who would join the Round Table, and that one of those sons would be connected to the name of the child who would destroy Arthur's realm.

Pellinore became one of Arthur's earliest and most valuable allies. He joined the king's wars against the rebellious vassals who refused to accept a boy-king, and his prowess in battle was decisive. At the Battle of Tarabel, also called Dimilioc, Pellinore fought with such fury that he struck a mighty blow at King Lot of Orkney, the husband of Morgause and the father of Gawain, Agravain, Gaheris, and Gareth. Pellinore's first stroke missed Lot and killed his horse, but as Lot fell, Pellinore struck again, cleaving through the king's helm and head down to the brows. The host of Orkney fled, and the battle was won, but the blood feud was born. Lot's sons would never forgive the man who killed their father, and the prophecy of Pellinore's own death began to appear in golden letters on the tomb of Sir Perin, a miraculous warning that Gawain would one day avenge his father with his own hands. Ten years after Gawain was knighted, he and his brothers ambushed Pellinore and slew him, the first great murder among the Round Table knights and the wound that would never heal.

Yet Pellinore's greatest tragedy was not his death but his blindness. During the feast of Arthur's wedding to Guinevere, a white hart and a white brachet ran through the hall, followed by Nimue, the Lady of the Lake, whose hound had been stolen. Merlin commanded Pellinore to pursue the thief and rescue the lady, and Pellinore armed himself and rode with the furious speed that was his nature. By a well in a forest valley, he saw a lady holding a wounded knight in her arms, weeping and begging for help. Pellinore saluted her and rode on, too fierce in his quest to tarry. He caught the thief, Hontzlake, killed him with a single blow, and returned with Nimue to Camelot. But on the way back, he passed the well again and found the knight and the lady both dead, their bodies eaten by wild beasts, all save the lady's head. His heart mourned with a sorrow he could not explain, and he bore the head to Arthur's court. Merlin then revealed the terrible truth: the dead lady was Pellinore's own daughter, Alyne, born to a woman he had once loved and forgotten. For not saving her, Merlin prophesied, Pellinore would one day find that in his hour of greatest need, his best friend would fail him. The prophecy was fulfilled when Gawain, whom Pellinore had trusted and fought beside, became his murderer.

Pellinore's legacy lived on through his children. His illegitimate son Tor was raised by a cowherd's wife and proved his knighthood at Arthur's court, where Merlin revealed his true parentage and Pellinore watched with pride as his son was knighted. His legitimate sons Aglovale, Lamorak, and Percival all became knights of the Round Table, and Percival became the first Grail hero, the pure knight who would achieve the Holy Grail before Galahad surpassed him. His daughter Dindrane, Percival's sister, became the Grail heroine, a servant of the sacred vessel who helped her brother and Galahad achieve the mystical objective. Pellinore was said to be of the royal line of Joseph of Arimathea, the man who brought the Grail to Britain, and in the Livre d'Artus he was called the Maimed King after being wounded by a holy spear for doubting the Grail's power. In the end, Pellinore stands as one of the most tragic figures in Arthurian legend, a knight of absolute prowess and absolute blindness, a father of heroes and a victim of vengeance, a man who hunted a beast he could never catch and who destroyed the one thing he could never replace: his own daughter's life.