Elaine first appears in the Prose Lancelot, the vast romance that forms part of the 13th-century Vulgate Cycle, where she is called Amite or Helizabel, the wisest and most beautiful woman in her father's realm. Her father was King Pelles of Corbenic, the Grail King, a wounded keeper of the Holy Grail who was descended from the line of Joseph of Arimathea, the man who had brought the sacred vessel to Britain. Pelles had received a prophecy that his daughter would conceive a son by Lancelot, and that this child would be named Galahad, would surpass his father in virtue, and would achieve the Holy Grail that had eluded every other knight. But Lancelot's heart belonged utterly to Queen Guinevere, King Arthur's wife, and he would never knowingly lie with another woman. The prophecy, it seemed, could not be fulfilled by honest means.

Elaine's first encounter with Lancelot was one of rescue. The enchantress Morgan le Fay, half-sister of King Arthur and the most powerful sorceress in Britain, had trapped Elaine in a bath of boiling water out of pure jealousy for her beauty. The spell could only be broken by the best knight in the world, and Lancelot, who had come to Corbenic in search of the Grail, freed her from the scalding torment. Elaine fell in love with him at once, but Lancelot gave her only courtesy and departure. He had no eyes for any woman but Guinevere. Undeterred, Elaine turned to Dame Brisen, her handmaiden and a sorceress of considerable skill. Together they devised a plan. Brisen gave Lancelot wine drugged with a potion that clouded his mind, and Elaine wore a ring that made her appear as Guinevere in his eyes. That night, Lancelot lay with the woman he believed to be his queen, and Elaine conceived the child who would change the Arthurian world forever.

When Lancelot woke and saw Elaine beside him instead of Guinevere, his fury was terrible. He drew his sword and would have killed her where she knelt, naked and trembling, had she not spoken the words that stayed his hand. She told him she was pregnant with his son, the child who would be the greatest knight in the world, the child who would achieve the Holy Grail. Lancelot lowered his blade. He did not forgive her, but he did not strike her. He kissed her once, a gesture of bitter acceptance, and departed. Elaine returned to her father's castle and gave birth to Galahad, raising him in the shadow of the Grail until the day he would come to Camelot and sit in the Siege Perilous, the deadly chair at the Round Table that had been waiting for him since before he was born. But Elaine's love for Lancelot did not end with his departure. It grew, as unrequited love does, into an obsession that would lead her to a second deception and to the ruin of the man she loved.

The second tryst came after Elaine brought the infant Galahad to Arthur's court. She appeared at Camelot with her son and with Dame Brisen, and once again the sorceress worked her magic. Lancelot was tricked into Elaine's bed a second time, believing again that she was Guinevere. But this time the deception was discovered. Guinevere, whose own chamber was nearby, heard them together and burst into the room in a jealous rage. She accused Lancelot of betrayal, and the shock and shame drove him from his senses. He ran from Camelot, tearing off his clothes, and wandered the wilderness for months, naked, filthy, and insane, eating grass and roots like a beast, his mind shattered by the loss of the queen he loved and the guilt of what he had done. It was Elaine who found him. She came upon him in the garden of Corbenic, raving and wild, and she had him carried to the tower where the Holy Grail was kept. The sacred vessel appeared, and its power restored Lancelot's mind, healing the madness that Guinevere's rejection had inflicted. But Lancelot did not thank Elaine for saving him. He saw her only as the instrument of his fall, and his heart remained with Guinevere.

After his recovery, Lancelot could not return to Camelot. The shame was too great, and Guinevere's anger too fresh. King Pelles offered him refuge on the Joyous Isle, also called the Island of Joy, and Elaine went with him. For two years they lived together, and Lancelot took the name the Chevalier Mal Fet, the Knight Mis-made or the Ill-Made Knight, a name that spoke to his broken state. He kept the island against all challengers, fighting any knight who came to test him, and Elaine was his companion in that exile. But it was not a marriage of love. Lancelot's heart was still at Camelot, and when his cousin Ector de Maris and his fellow knight Percival came to the island and persuaded him to return, Lancelot left without hesitation. Elaine was left behind, and the grief of his departure was more than she could survive. When Lancelot returned to Corbenic during the Grail Quest, he learned that Elaine had died in the interim, her heart broken by the knight who had given her a son and taken everything else away. In Thomas Malory's Le Morte d'Arthur, she is the Grail Maiden who appears in the halls of Corbenic with the sacred vessel, the mother of the perfect knight, and the woman whose love, however obtained, was genuine enough to heal a madman and raise a saint. She should never be confused with Elaine of Astolat, the Fair Maid who died for love of Lancelot and floated downriver in a white barge, a different tragedy with the same name.