Constans was the eldest son of the usurping Emperor Constantine III and was a Roman Caesar under his father.

Constans lived in a monastery for the younger years of his life until he was elevated to the title of Caesar. His father then sent him with the general, Gerontius, towards Spain. This was to destroy factions of the royal family who were still living and causing disturbances in the western empire. Constans left his wife and household at Zaragoza under the care of Gerontius to return to report to Arles. By September of 409, the barbarians who had penetrated the Rhine defenses, and had spent the intervening two years roaming and plundering their way through Gaul, had reached the Pyrenees, where they broke through Constantine's garrisons and entered Spain. While Constantine prepared to send his Constans back to deal with this crisis, word came that his general Gerontius had rebelled, raising his own man as co-emperor. Despite Constantine's best efforts, his fear of an attack from Spain did come to pass in the following year, when Gerontius advanced with the support of barbarian allies. Constantine's forces facing the rebel, Gerontius, were defeated at Vienne (411), where Constans was captured and executed.

The Legend

British legend, aided by Geoffrey of Monmouth, states that Constans was elected by the Britons as king following Constantine's death. This utterly contradicts the previous known history of Constans, as does the remainder of his British story. It is said that Vortigern, the leader of the Gewissei tribe of Celts, supported Constans as king, although he, too, wanted such a rank. Constans lived the life of a monk until Vortigern convinced him to be crowned king. This seemed merely ceremonial for soon after, Vortigern took over all powers of the kingship upon himself and used Constans merely as a puppet. He was killed by rebellious Picts while sleeping in his bedroom one night. These Picts were influenced highly by Vortigern, who sought above all else to be king. And this he did, for Constans was succeeded by Vortigern.