Severus II, who reined only for a short time had a modest family background and came from Illyricum, probably around the Danubian region. Already early in his career he had held a military command. Severus II had quickly risen through the ranks of the army like most of the Danubian leaders at the time. In AD 305 he was appointed western Caesar by Constantius I Chlorus when he ascended under the system of government instituted Tetrarchy, where Diocletian, at Nicomedeia, and Maximianus Herculius, at Mediolanum were replaced with Constantius I and Galerius as Augusti. Maximus Daia became the eastern Caesar. In the same year Severus adopted the names "Flavius" in honor of the adoption of Constantius and "Valerius" in honor of being now an adopted member of the House of Diocletian. Part of Severus duty as western Caesar was to administer the provinces of Italy, Africa and Pannonia.
The appointments of Severus II and Maximinus Daia was for many at the time a big surprise. The two Caesars were regarded as unknown, and the general assumption was that the sons of Constantius Chlorus and Maximian would accede to the throne. In AD 306, Only one year later, the western Augustus Constantius I Chlorus died. Galerius made Constantius' son Constantine Caesar and raised Severus to the rank of Augustus to replace the dead Constantius Chlorus, but instead of a smooth transition, the British troops put Constantius' son Constantine on the throne. Galerius, the senior Augustus in the East was furious at this development but recognized Constantine rather than risk civil war. Things went even worse when Maxentius, the son of Maximianus Hersulius started a revolt because of Galerius tax policy and was proclaimed emperor in Rome at the end of October AD 306.
As soon as Galerius heard the news of the revolt, he asked his co-emperor Severus to leave the capital Mediolanum and to march with a large field army towards Rome to put down this rebellion. Severus who was well supported in northern Italy (the rest of the Italian peninsula and Africa though declared in favour of Maxentius) ran nevertheless soon into trouble. Because when Maxentius learned about the advance of Severus he sent his own father Maximianus the purple and offered him to be Augustus again. The idea was that once Severus army heard the news they would switch to his side. The plan worked, Severus II's troops had no loyalty towards Maxentius, but they still felt strongly for their old emperor Maximian. In addition Maxentius bribed Severus soldiers during the siege of the city, and Maximians' agents had little problem causing trouble among the army.
Severus' army, torn between their loyalty to two emperors then mutinied. When even Severus' praetorian prefect turned against him and started promising bonuses to men who would change their allegiance, Severus' fate was sealed. With little else to do, he withdrew quickly and fled with his remaining loyal men to Ravenna. Maximian pursued him with his troops, caught up with him in Ravenna, and with false promise of safety convinced Severus to surrender.
Severus agreed to abdicate if his life was spared. Severus II was then taken first under house arrest and soon after brought to Rome where he was put to prison in Tres Tabernae after he was paraded through the streets of Rome. The holding of Severus was thought to provide safety against attacks from Galerius. He, however, attacked anyway but he to had to realize just how dangerous it was to advance his troops against Maximian, a man whose authority many of the soldiers respected more than his own. With many of his forces deserting, Galerius had to simply withdraw. With no need for a hostage anymore, Maxentius then ordered to put Severus to death. Severus II died on the 16 September AD 307.