This Irish knight was the son of King Morhaut Senior, brother of Queen Isolde of Ireland and father of Sir Golistant. He was an early adherent of King Arthur, brought into the fold by Sirs Gawain and Yvain. These two discovered his white shield one day, hanging from a tree and being vandalized by twelve maidens who claimed that its owner was a misogynist. Morhaut then returned, recovered his despoiled shield and drove the ladies back to the tower from whence they had come. He greeted Gawain and the two took part in a friendly joust together. Morhaut won because he grew stronger as the evening approached. He eventually explained that the twelve maidens were wicked enchantresses and that, like any good knight, he would give good service to any lady who asked for it. Morhaut, Gawain and Yvain then trav­elled together for a time.

Upon reaching Arroy (perhaps Ayrshire), the companions met the Damsels of Spring, Summer and Autumn and Morhaut chose to seek adventure with Summer for the next year. With his new partner, he travelled to the South Marches, where Morhaut was challenged to a dual by the ruling Duke. The nobleman had hated the Knights of the Round Table ever since Sir Gawain had slain seven of his sons, but when Morhaut defeated him, he quickly offered his allegiance to King Arthur. Morhaut next attended the Lady de Vawse’s Tournament and defeated some forty knights to win a circlet of gold. Then it was on to the lands of Earl Fergus, where he rid his lordship of a giant named Taulurd. He also rescued the Lady of Plessis from being burnt alive, but less honourably was captured and thrown in prison at the Rock of Maidens (probably Edinburgh) until released by Sir Gaheris. At the year’s end, Sir Morhaut set off to rendezvous with Gawain and Yvain once, jousting and defeating Sirs Sagramore, Osanna, Dodinel and Meliot on the way. The three friends were overjoyed to be reunited and returned to Arthur's Court in great triumph. Sir Morhaut was sponsored as a Knight of the Round Table and enrolled on the same day as Sir Pelleas.

Joining Lancelot, Bors, Kay and Marrok, Morhaut became one of the King Arthur's personal bodyguard during his war with the Roman Emperor Lucius. He also became Royal Champion to his brother-in-law, King Anguish of Ireland. A less prestigious period in his life was his second imprisonment, this time by Sir Turquine, though he was eventually rescued by Sir Lancelot.

As Champion of Ireland, Morhaut was given the unfortunate task of travelling to Cornwall in order to force King Mark to pay his overdue tribute money to the Irish Exchequer. He was placed in single combat against the then very green Sir Tristram on Saint Samson's Isle. Not wanting the dishonour of killing an inexperienced knight, he kindly suggested that the Cornishman throw in the towel before the battle had begun. In the event, however, the ferocious clash lasted all the day until finally Sir Tristram struck his sword right through Morhaut’s helm and embedded it in his skull. Sir Morhaut just managed to stagger to his feet, but barely made it back to his ship and the safety of Ireland. Despite the great efforts of his sister, he died a few days later with a piece of Tristram's sword still protruding from his head. It was perhaps fitting that when Tristram was made a Knight of the Round Table, he was given the seat of this unfortunate knight whom he killed.