Maximian was born near Sirmium at around AD 250 to a poor shopkeeper's family. He received little or no formal education at all.
It was at this accession that Maximian adopted the names Marcus Aurelius Valerius. His names given to him by birth, other than Maximianus, are unknown.
Had Diocletian raised Maximian in order to free his own hands to deal with urgent military matters along the Danube, this left Maximian to quell the troubles arising in the west. In Gaul the so-called bagaudae, robber bands made up of peasants driven out of their homes by invading barbarians and army deserters, rose up against Roman authority. Their two leaders, Aelianus and Amandus, may have even proclaimed themselves emperors. But by the spring of AD 286 their revolt had been crushed by Maximian in several minor engagements. Shortly after, his troops, prompted by Diocletian, hailed Maximian Augustus on 1 April AD 286.
It was a strange choice by Diocletian to make Maximian his colleague, as the accounts describe Maximian as a coarse, menacing brute with a savage temper. No doubt he was a very capable military commander, a skill of high priority for a Roman emperor. But one can't help but feel that not merit but Maximian's long standing friendship to the emperor and not least his origin, being born so close to Diocletian's place of birth, will have been deciding factors.
The following years saw Maximian repeatedly campaigning along the German frontier. In AD 286 and 287 he fought off invasions by the Alemanni and the Burgundians in Upper Germany.
However, in the winter of AD 286/7 Carausius, the commander of the North Sea fleet, based at Gesoriacum (Boulogne), rebelled. Controlling the Channel fleet it wasn't especially hard for Carausius to establish himself in Britain as emperor.
When Diocletian established the tetrarchy in AD 293, Maximian was allotted control of Italy, the Iberian peninsula and Africa. Maximian chose his capital to be Mediolanum (Milan).
The year AD 303 saw a harsh persecution of the Christians throughout the empire. It was initiated by Diocletian, but executed in agreement by all four emperors. Maximian exacted it especially in north Africa.
Then, in the autumn of AD 303, both Diocletian and Maximian celebrated together in Rome. The cause for the grand festivities was Diocletian's twentieth year in power.
Though when early in AD 304 Diocletian decided that they both should retire, Maximian was unwilling.
The abdication of the two Augusti had now transferred their power to Constantius Chlorus and Galerius, who in turn promoted Severus II and Maximinus II Daia to their places as Caesars.
Using a mixture of persuasion and force Maximian then successfully used his forces and influence to repel both Severus II and Galerius in their attempts to march on Rome. Next he travelled to Gaul where he created a useful ally by marrying his daughter Fausta to the son of Constantius Chlorus, Constantine.
Alas, in April AD 308, Maximian then turned on his own son Maxentius. Whatever the causes might have been for this bizarre turn of events, Maximian reappeared in Rome amidst much drama, but his attempt to win over his son's soldiers failed, which forced him to withdraw back to Constantine in Gaul.
A council of the emperors then was called by Galerius at Carnuntum in AD 308. At the conference not only Maximian, but so too Diocletian was present. Despite his retirement, it apparently still was Diocletian who possessed greatest authority in the empire.
But there once more his ambition got the better of him and he pronounced himself emperor for the third time in AD 310, while his host was campaigning against the German on the Rhine. Though Constantine immediately wheeled his troops around and marched into Gaul. Maximian had obviously not calculated for any such swift response from Constantine. Taken by surprise, he was unable to make the necessary preparations for a defence against his new enemy. And so all he could do was flee southward, to Massilia (Marseille). But there was no stopping Constantine. He laid siege to the city and forced its garrison to surrender. Maximian was handed over the surrendering troops.