Sir Gareth was one of the sons of King Lot of Orkney and Queen Morgause. He was somewhat younger than his brothers and was thus left behind in Orkney when they travelled south to join King Arthur’s Court. Years later, however, Gareth followed them in disguise. Arthur placed him under the tutelage of Sir Kay who made fun of the lad and put him to work in the kitchens. He gave him the nickname 'Beaumains' - Soft Hands - because the young prince had never done a day’s work in his life. When the Lady Lynette came to Camelot in search of a knight to champion her sister, Lyonesse, against Sir Ironside the Red Knight who was besieging her Castle Perilous. Gareth volunteered and Lancelot knighted him ready for the task. Kay tried to protest, but Gareth beat him in single combat. Lynette was most put out at being given a kitchen-hand to help her and showed him only contempt all the way home. However, Gareth was triumphant over not only Sir Ironside, but also his brothers, the Black, Green and Blue Knights. Sir Gareth and Lady Lyonesse subsequently fell in love and married at Kynke Kenadonne.

Gareth was active during King Arthur's war against the Roman Emperor Thereus and killed King Datis of Tuscany. He proved himself to be more admirable than any of his brothers: preventing Gawain and Agravaine from killing Gaheris after he beheaded their mother while in a state of shock; condemning them for the murder of her lover, Sir Lamorak; and pleading with Agravaine and Mordred not to reveal Guinevere and Lancelot’s affair. On the last occasion, he was ignored and his brothers’ actions led to his death at the hands of Sir Bors during Sir Lancelot’s rescue of Queen Guinevere from the stake.

In origin, Gareth appears to have been a little known British character named Gwalchafed, a son of King Lot Luwddoc of Gododdin.