Sir Galehaut was the son of Sir Brunor of Castle Pleure on the Giant's Island (possibly Lundy) and his wife, Bagotta. Rather than inherit his father's domain, family tradition insisted that he conquer lands for himself. Thus Galehaut overran some thirty island kingdoms, of which Sorelais (possibly the Isle of Man) and the Distant Isles (the Hebrides) were his favourites.

Galehaut started out as an adversary of King Arthur. As a strong monarch himself, he wanted Arthur to recognise his overlordship. Great battles ensued but, eventually the disguised Sir Lancelot won Galeholt over and he pledged his allegiance to Arthur. Galehaut thus became a great friend of Lakeland Knight and was his confidant during his rapturous affair with Guinevere. The Queen confided, similarly, with the Lady of Malohaut, and as both she and Galehaut acted as go-betweens they soon became lovers themselves. During the "false Guinevere" episode, Galehaut hid the lovers away at Sorelais.

In order to better himself, King Galehaut was keen to mix with the Knights of the Round Table and he eventually became one of their number. His ally, King Bagdemagus of Gore acted as regent while he was away from his own kingdoms. Galehaut is perhaps best known for the great tournament he held at Sorelais, though Malory says this was organised in order to slay Sir Lancelot which somewhat contradicts the story that the two were great friends. Such friends, in fact, that Galehaut could not believe his sages when they interpreted a disturbing dream to mean that Lancelot would be the cause of his death. However, during Lancelot’s maddened wanderings, Galehaut received a false report that his friend was dead. Distraught and depressed, the King shut himself away and died of grief. Lancelot retrieved his body from its monastic burial place and had him re-interred at Joyous Garde.