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 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 after Constantine's death. This contradicts the known history of Constans, as does the remainder of his British story. The legend claims Constans, older brother to Aurelius Ambrosius and Uther Pendragon, spent his early life studying at a monastery. During the succession crisis following Constantine's murder, Vortigern, leader of the Gewissei tribe of Celts, convinced Constans to leave the religious life and become king. The weak and unpopular former monk became a puppet, putting all but the title of his office into the hands of Vortigern. Vortigern eventually disposed of him by tricking some Picts, liaisons in his employ, into murdering him in his sleep. The treacherous Vortigern took the throne for himself.