Constantius III was a Roman citizen born at Naissus at an unknown date.
His rise to power came at a time of desperate weakness by the western empire. Alaric had just sacked Rome in AD 410. His brother-in-law Athaulf still remained in southern Italy at the head of the Visigoths.
When Gerontius moved into Gaul, killed Constans and laid siege to Constantine III in Arelate (Arles), Constantius III marched into Gaul himself and drove Gerontius back into Spain, laid siege to Arelate himself and captured the city with Constantine III, who was executed shortly after. Gerontius troops mutinied in Spain and murdered their leader, with the puppet emperor Maximus being deposed and exiled in Spain.
After this Constantius III moved back down into Italy and drove Athaulf and his Visigoths out of the peninsula into Gaul in AD 412.
Meanwhile a deal was struck with Athaulf who defeated a new would-be emperor in Gaul named Jovinus.
In AD 414 though Athaulf at Narbo (Narbonne) married Galla Placidia, the half-sister of Honorius whom Alaric had taken hostage during his sack of Rome in AD 410. This angered Constantius III who had had his own designs on Placidia. Furthermore Athaulf now set up a puppet emperor of his own in Gaul, Priscus Attalus who had already been a puppet emperor for Alaric in Italy.
Constantius III marched into Gaul and forced the Visigoths into Spain and captured Attalus who was paraded through Rome. Athaulf then was murdered and his brother and successor, Wallia, handed Placidia back to Constantius III whom she reluctantly married on 1 January AD 417.
Under Wallia the Visigoths agreed to wage war against other German tribes (Vandals, Alans, Sueves) in Spain for the Romans and were in AD 418 granted the status as federates (independent allies within the empire) and settled in Aquitania.
Constantius III had in effect brought back the western empire from the very brink of disaster. He had governed the western empire for ten years and been Honorius' brother-in-law for four, when in AD 421 Honorius was persuaded (much against his will supposedly) to reward him by raising him to the rank of co-Augustus of the west. His wife, Aelia Galla Placidia, also received the rank of Augusta.
Theodosius II, emperor of the east, though refused to accept these promotions.
But after only seven months of rule as emperor, Constantius III, suffering from a decline in health, died in AD 421.