Constantius was a soldier in the Roman army who became a successful general and was adopted into the family of the emperor Maximian by his marriage to Maximian's daughter, Theodora. It is unclear whether Constantius had to first divorce his forst wife, Helena, in order to marry Theodora, or whether Helena and Constantius were never married in the first place.

In 293, Constantius was named by Maximian as his Caesar, while Diocletian chose Galerius. This established the Tetrarchy, with Maximian and Constantius governing affairs in the West and Diocletian and Galerius governing in the East. One of Constantius's most important tasks was to put down the revolt of Carausius in Britannia, which he achieved, causing the assassination of Carausius by Allectus, who was in turn defeated and killed by the armies of Constantius in 295, returning Britannia to Roman control.

In 305, Diocletian retired, and forced Maximian to retire too. This left Constantius as Augustus in the West, and Galerius in the East. As Caesars, Constantius had Severus and Galerius had Maximinus Daia.

Constantius died in 306. His son by Helena, Constantine, was declared his successor by his troops.

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