In Nennius's Historia Brittonum, Vortigern's eldest son, brother of Catigern, Faustas, and Pascentius.

He breaks with his father and makes war on the Saxons he has settled in Kent. Vortimer fights them four times and drives them out but dies soon after from his wounds received at the fourth battle. On his deathbed he asked a monument, a statue, to be erected, but his barons failed to carry out the request. According to Geoffrey of Monmouth, his countryment did not comply. The Saxons return with Vortigern's acquiescence and resume their depredations.

Geoffrey expands the story, saying that Britons, angry at Vortigern's pro-Saxon stance, deposed him and made Vortimer king instead. He proved to be a much nobler king than his father. He led Briton armies against Hengist and the Saxons, fought four battles against them. He pushes the Saxons to the isle of Thanet, after which they surrendered and returned to Germany. When he won the Kentish victory, his Saxon stepmother, Rowena, poisoned him, and Vortigern was back in power. When the Saxons heard of Vortimer's death they returned in force.

Layamon says that during his reign, and at his request, two bishops - Germanus and Louis - were sent to Britain to restore Christianity. He was buried in either Lincoln or London. Triad 37 avers that his bones were buried in the chief British ports. Other traditions suggests that a statue of him was put up at Dover. Commentators such as Brodeur regard Vortimer as a purely fictional character.

The battles are variously given to have occured at Darenth or Derwent, at Episford or Aylesford, by the Inscribed Stone, along the seashore in Kent, and at other unnamed locations.

As a figure of history, Vortimer eludes definition. If he existed at all, he probably did die before Vortigern, because there is no hint of a succession. Morris, one of the few modern historians to take him seriously, equates three of his battles with three recorded in the Anglo-Saxon Chronicle and dated 455, 457, and 465. Since Vortigern is unlikely to have lived beyond the mid-450s, this attempt to pin Vortimer down involves pulling back the Chronicle dates ten years or more. Geoffrey puts his campaign earlier still - contemporary with the mission of Germanus and Lupus in 429. This, however, is a product of his dynastic fictions. In 429, any Saxons settled in Britain as auxiliary troops would still have been few and controllable.

When he writes of Vortimer, Nennius employs the Welsh spelling Guorthemir. This has variants of its own, but the fifth-century original from which it derives as a British form Vortamorix, which, like Vortigern, would probably have been a title or designation. The syllable vor meant "over"; tamo- was a superlative suffix; rix meant "king". As Vortigern is the "over-king", so Vortimer is the "over-most" or "highest" king. Its etymological closeness to "Riothamus" or Rigotamos, the designation of a king who was certainly real, may be thought to favor Vortimer's own reality.

Whether or not he was actually Vortigern's son, he may have put up a rival claim to authority, adopting a distinctive and grander form of the royal style.