Constantine was born into the royal house of Dumnonia, the kingdom of the West Country that covered modern Cornwall and Devon. His father was Cador, the Duke of Cornwall, a powerful warlord whom Welsh tradition remembers as Cadwy mab Geraint and whom Geoffrey of Monmouth calls Arthur's cognatus, his blood relative. The exact kinship is never specified - some later romances make Cador the son of Arthur's unnamed sister, which would make Constantine Arthur's grand-nephew, while others simply call him cousin - but the bond was close enough that Arthur trusted him above all other men when the kingdom needed a steward. In the 6th century, the historical Constantine ruled only in Dumnonia, a petty kingdom in a land of crumbling Roman villas and resurgent tribal warfare, but the genealogies of the Dumnonian kings name him as the father of Erbin and the grandfather of Geraint, the hero of the romance Geraint and Enid, giving his bloodline a reach that extended far beyond his own lifetime.

The earliest record of Constantine is also the most damning. The monk Gildas, writing his furious sermon De Excidio et Conquestu Britanniae around 540 CE, names Constantine as one of five Brittonic kings whom he compares to the beasts of the Apocalypse. Gildas calls him the tyrannical whelp of the unclean lioness of Damnonia, a biblical slur that condemned not only Constantine but his mother as well. The monk accuses him of casting off his lawful wife, committing many adulteries, and then - the sin that Gildas finds most unforgivable - disguising himself in an abbot's robes to attack two royal youths as they prayed before a church altar, killing them and their companions in the very sanctuary that should have protected them. Gildas knows Constantine is still alive as he writes, and he warns the king to repent before divine judgment falls. The murdered youths may have been rival claimants to the Dumnonian throne, or they may have been the Saints Dredenau, two young princes later venerated in Brittany whose cult grew up around the memory of their murder by an ambitious uncle.

The transformation of this condemned Dumnonian warlord into the heir of King Arthur is the work of Geoffrey of Monmouth, who composed his Historia Regum Britanniae around 1136. Geoffrey took the raw material of Gildas and the Dumnonian genealogies and recast Constantine as Arthur's successor rather than one of five contemporary tyrants. In Geoffrey's version, Arthur is mortally wounded by Mordred, the traitor nephew who had seized the queen and the throne while Arthur was fighting the Romans in Gaul. At the battle of Camlann, Arthur and Mordred strike each other down, and as the king lies dying he passes the crown of Britain to Constantine, the son of Cador. The only other survivor of that terrible day is Constantine himself, who stands amid the corpses of the Round Table to receive a kingdom that has already been destroyed. Geoffrey identifies Gildas's two royal youths as the sons of Mordred, who continue their father's rebellion after his death, allied with Saxon invaders. After many battles Constantine defeats them, and the boys flee to London and Winchester, hiding in a church and a friary respectively. Constantine hunts them down and executes them before the altars of their sanctuaries, repeating the very sin that Gildas had condemned - the murder of royal youths in holy places - but now doing it in the name of avenging Arthur's death.

Thomas Malory's Le Morte d'Arthur, written in 1485, gives Constantine a more honourable role in the tragedy. Malory makes him a Knight of the Round Table and Arthur's next of blood, and before the king rides out to face the Romans he names Constantine and Sir Baudwin of Britain as joint governors of the realm. When Arthur is wounded at Camlann, he appoints Constantine as his heir in a loud voice before all his men, and after the king is borne away to the Isle of Avalon, Constantine is crowned King of England. Malory reports that he was a good king, that he restored the Archbishop of Canterbury to his diocese, and that he brought order back to a land that had seen its finest knights slaughtered or scattered to hermitages in Gaul. This gentler version reflects Malory's desire to end his great book on a note of hope, to suggest that the ideals of the Round Table could survive even the death of the king who had created them. But Malory does not forget the darker tradition entirely; he knows that Constantine's reign was brief and that the kingdom he inherited was too broken to save.

The end came three years after Camlann, and it came by the same hand that had struck down the sons of Mordred. Geoffrey of Monmouth records that Constantine was killed by his nephew Aurelius Conanus, a figure derived from Gildas's Aurelius Caninus, the lion-like king who had been one of the five tyrants alongside Constantine himself. The murder precipitated a civil war, and Constantine was buried at Stonehenge alongside the other kings of Britain, his bones laid among the great stones that Merlin had transported from Ireland generations before. But Constantine's story did not end with his death. In the hagiographic tradition of southwest Britain, a Saint Constantine emerged - perhaps the same man, perhaps a conflation with other holy figures - who converted to Christianity through the intervention of Saint Petroc, abdicated his throne to become a monk, founded churches across Cornwall and Devon, and was eventually martyred at Kintyre in 576. The Welsh romance Culhwch and Olwen preserves yet another Constantine, the shepherd of Ysbaddaden Chief Giant and the father of Goreu, a figure who may be a Welsh prototype of the Arthurian heir. Whether these Constantines are one man fractured by time or many men compressed by legend, the name endures as the bridge between the historical darkness of sub-Roman Britain and the luminous tragedy of Arthur's fall.