He originally followed his father’s occupation, that of a herdsman, where he got his surname of Armentarius (Lat. armentum, herd). He served with distinction as a soldier under Aurelian and Probus, and in 293 was designated Caesar along with Constantius Chlorus, receiving in marriage Diocletian’s daughter Valeria, and at the same time being entrusted with the care of the Illyrian provinces.
In 296, at the beginning of the Persian War, he was removed from the Danube to the Euphrates; his first campaign ended in a crushing defeat, near Callinicum, but in 297, advancing through the mountains of Armenia, he gained a decisive victory over Narses and compelled him to make peace.
In 305, on the abdication of Diocletian and Maximian, he at once assumed the title of Augustus, with Constantius his former colleague, and having procured the promotion to the rank of Caesar of Flavius Valerius Severus, a faithful servant, and Daia (Maximinus), his nephew, he hoped on the death of Constantius to become sole master of the Roman world.
After an unsuccessful invasion of Italy in 307, he elevated his friend Licinius to the rank of Augustus, and, moderating his ambition, devoted the few remaining years of his life "to the enjoyment of pleasure and to the execution of some works of public utility."
It was at the instance of Galerius that the first of the celebrated edicts of persecution against the Christians was published, on the February 24, 303, and this policy of repression was maintained by him until the appearance of the general edict of toleration, issued in 311 in his own name and in those of Licinius and Constantine. He died in May, 311.