Lionesse was born into a noble family of great lands and greater peril. Her brother was Sir Gringamore, a knight of Avalon, and her sister was Lynette, the damsel who would one day ride to Arthur's court to beg for a champion. Lionesse herself held Castle Dangerous, a stronghold so named because it had become a trap for the knights who tried to defend it. The Red Knight of the Red Lands, whose true name was Sir Ironside, had laid siege to her castle and hung the bodies of forty defeated knights on a great sycamore tree outside the walls, their shields round their necks and their spurs on their heels. The Red Knight's strength waxed with the morning sun until noon, making him seven times as strong as a normal man, and no champion who had faced him had survived. Lionesse was trapped within her own walls, watching from a window as each new rescuer fell, until her sister Lynette set out for Camelot to find the one knight who might succeed where all others had failed.

The defining encounter of Lionesse's life came not with the Red Knight but with the kitchen page who defeated him. When Lynette arrived at Arthur's court, she refused to give her name or her sister's name, and in punishment for her silence she was given not Sir Lancelot or Sir Gawain but a young man called Beaumains, who had spent a year working in Arthur's kitchen. This was Gareth of Orkney, the youngest son of King Lot and Queen Morgause, Arthur's own nephew, though no one at court knew it except Lancelot, who had knighted him in secret. On the journey to Castle Dangerous, Lynette abused Gareth constantly, calling him kitchen-knave and scullion, but Gareth endured it with the patience that would prove his chivalry. When they finally reached the castle and Lionesse looked down from her window, she made a curtsey to the unknown knight and held up both her hands, and Gareth knew at once that he would fight to the death for her.

The battle with the Red Knight was one of the longest and bloodiest in Malory's entire book. The two knights fought from morning until evening, their horses killed, their shields shattered, their armour torn away in great pieces. At noon, when the Red Knight's strength was at its peak, Gareth was driven to the ground, and Lionesse wept at her window while Lynette shouted down to the fallen knight: "O Gareth, have you lost your courage? My sister weeps and breaks her heart, because her true knight has fallen." Those words restored him. He leaped to his sword and fought with renewed fury, and by evening the Red Knight lay defeated at his feet. Gareth spared the knight's life on condition that he yield to Lionesse and make amends for all he had stolen, and the Red Knight went to Arthur's court to tell the king what Sir Gareth had done.

What followed was one of the most unusual courtships in Arthurian legend. When Gareth came to Castle Dangerous expecting to be welcomed as a hero, he found the drawbridge raised and armed men barring the gate. From her window, Lionesse told him that she could not love him altogether until he had been King Arthur's knight for another year and helped to clear the land from his enemies. She promised that she would love him all her life and that his labour would not be in vain, but she sent him away. Gareth went, sad but faithful, and spent the year proving himself. When the time came, Lionesse arranged a great tournament at her castle on the feast of the Assumption, and she gave Gareth her magic ring, which changed all the colours of his apparel and protected him from wounds, so that he could fight in disguise against the Round Table. The ring enabled him to come and go unseen, but his dwarf tricked it away from him for a moment, and in that moment his secret was revealed. Gareth fought his own brother Gawain in the lists, and when the heralds finally acknowledged that the unknown knight was Sir Gareth of Orkney, the tournament became a celebration of recognition.

The wedding of Lionesse and Gareth was one of the great set pieces of Malory's book, a joint ceremony at Michaelmas in which three couples were married at once: Gareth and Lionesse, Gaheris and Lynette, and Agravain and Laurel, Lionesse's niece. Every knight Gareth had defeated or saved was present, and the feast lasted for days. But the shadow of tragedy already hung over the marriage. Lionesse had tried twice to come to Gareth's bed before the wedding, and both times Lynette had intervened with magic, conjuring an enchanted knight who stabbed Gareth in the thigh to prevent the consummation. Lynette healed the wound with a special ointment and stuck the knight's severed head back on with her craft, determined to keep her sister pure until the proper nuptials. These premonitions of violence would prove prophetic. Years later, when Lancelot rescued Guinevere from the stake, he would strike down Gareth accidentally in the chaos, and Gareth's death would turn his brother Gawain against Lancelot and set in motion the final destruction of the Round Table. Lionesse, the lady who had waited a year for her knight, would become one of the widows of Camelot, her magic ring powerless against the tragedy that no enchantment could prevent.