Of Illyrian descent, Constantius had a distinguished military career before serving as governor of Dalmatia (in modern Croatia). In 289 he left his concubine, Helena, mother of Constantine, and married Theodora, the stepdaughter of the emperor Maximian; with Theodora he had three children, Dalmatius, Constantius, and Constantia. On March 1, 293, Constantius was adopted by Maximian and made his caesar. The two men, together with Diocletian and his caesar, Galerius, formed the tetrarchy.

Constantius was assigned to rule Gaul and ordered to subdue Marcus Aurelius Carausius (Carausius, Marcus Aurelius Mausaeus), a usurper in Britain. In 293 he captured Carausius's mainland base, Gesoriacum (modern Boulogne, France). Allectus, Carausius's finance minister, murdered him and was granted three years of peaceful rule. Then in 396 Constantius and his praetorian prefect, Asclepiodotius, launched a brilliant two-pronged attack on Britain. Asclepiodotius defeated and slew Allectus in Hampshire, while Constantius sailed up the Thames to London and destroyed the remnants of Allectus's army. Constantius then set about restoring frontier defenses. He took strong measures to eliminate Frankish and Saxon piracy, and in 298 he triumphed over the Alemanni in Gaul. His enforcement of Diocletian's edicts (303) against the Christians was deliberately lax; he demolished some churches but did not execute believers. When Diocletian and Maximian abdicated on May 1, 305, Constantius became the senior emperor in the West. He died the following year in a victory over the Picts, and his troops proclaimed Constantine emperor. (Constantius's nickname Chlorus, meaning “The Pale,” is first found in Byzantine sources.)