Little is known about the early life of Postumus, but it is believed that he was a Gaul of humble origins who rose through the ranks of the army, eventually becoming the governor of Upper or Lower Germany. While Gallienus was dealing with problems in the east, he left his son, Saloninus[?], and military commanders, including Postumus, to protect the Rhine. Amid the chaos of an invasion by the Alemanni and Franks, Postumus was declared emperor. Postumus then besieged and attacked Cologne where Silvanus, praetorian prefect and former co-director of Roman policy on Gaul (along with Postumus) had sided with Saloninus. After breaching the walls of the city, Postumus had Silvanus and Saloninus killed; later he erected a triumphal arch to celebrate his victory.
He was recognized as emperor in Gaul, Spain, Germany, and eventually Britain. Postumus set up the capital of his renegade empire at Trier, complete with its own senate, consuls and praetorian guard. He represented himself as the restorer of Gaul on some of his coins, a title he earned after successfully defending Gaul against the Germans. The coins issued by Postumus were of better workmanship and higher precious metal content than coins issued by Gallienus.
In 263, Gallienus launched a campaign to defeat Postumus. After initial success against Postumus, Gallienus was seriously wounded and needed to return home. After his failed attempt at defeating Postumus, Gallienus was occupied with crises in the rest of his empire and never challenged Postumus again.
Aureolus[?], a general of Gallienus who was in command of Milan, openly changed sides and allied himself with Postumus. The city of Milan would have been critical to Postumus if he planned to march on Rome. For whatever reason, Postumus failed to support Aureolus, who was besieged by Gallienus.
Postumus, a usurper of Gallienus, was himself challenged by a usurper in 268. Laelianus[?], one of Postumus' top military leaders, was declared emperor in Mainz by the local garrison and surrounding troops. Although Postumus was able to quickly besiege Mainz and kill Laelianus, he was unable to control his own troops and they turned on him and killed him. There are two different explanations for why his troops turned on him. One holds that his troops were dissatisfied with him for not allowing them to sack the city of Mainz. The other proposes that it was supporters of Laelianus among his own troops that turned on him.
NOTE: Although his reign is often listed as beginning in 259 AD, it is now believed that the summer or fall of 260 AD is the more likely date that he was hailed emperor. This topic is still hotly debated. If the date of 260 is chosen for the start of Postumus' reign, then all subsequent dates involving the Gallic Emperor are pushed back by one year.