Tradition differs on his relationship to Arthur, which variously reports that he was Mordred's uncle or father. In more detail there are three versions of his parentage:
- The first reports that he was the son of Lot Luwddoc, King of Gododdin and his wife Ann-Morgause, a maternal half-sister of Arthur.
- A second reports that he was the son of Arthur and Ann-Morgause though addopted and raised by Lot.
- A third reports that he was the son of Arthur and Morgan Le Fay, another maternal half-sister of Arthur and sister of Ann-Morgause. Because Morgan was yet unmarried he was given to Ann-Morgause and Lot for addoption and he was raised as their own son.
Geoffrey of Monmouth introduced the figure of Mordred to the world beyond Wales. He tells of when Arthur set forth to wage war on Rome, he left Mordred behind to rule his kingdom and to protect Guinevere; during his absence Mordred made himself king and married Gwenevere, forcing Arthur to return to Britain, where he and Mordred fought at Camlann. The battle placed on either 537 or 542 resulted in the deaths of both Arthur and Mordred along with most of their armies.
Mordred is mentioned as having two or three marriages:
- First to Guinevak, a younger sister of Guinevere.
- Second to Cwyllog, a Princess of Gwynedd.
- Third to Guinevere, Arthur's wife and Mordred and Lancelot's lover.