Magnentius was born at about AD 303 around Samarobriva (Amiens) to an English father and a Frankish mother.
He served in a barbarian unit under he rule of Constantine. During the reign of his sons Magnentius then rose to become staff officer and then a commander of a mobile force (comitatenses), eventually receiving command of the elite legions known as the Joviani and Herculiani.
Constans, under whom Magnentius came to serve in the west, was deeply unpopular with the troops as well as ruling his part of the empire as a tyrant.
On 18 January AD 350 Magnentius made his bid for power, appearing in the imperial purple robes at a birthday party held by Marcellinus, the finance minister of Constans, at Augustodunum (Autun). It was there Magnentius was acclaimed Augustus.
The army at once changed their allegiance away from the hated Constans to Magnentius.
Constans fled, trying to reach Spain, but he was caught up with by Gaiso, one of Magnentius' agents.
With the last remaining son of Constantine the Great, Constantius II, occupied in the east, Magnentius remained untroubled, but for a challenge by the nephew of Constantine the Great, Nepotianus, in June AD 350 declared against Magnentius and instead proclaimed himself emperor.
At first Nepotianus, who was the son of Constantine's sister Eutropia, successfully defeated a force led by the praetorian prefect Anicetus, but only four weeks later he and his mother Eutropia were eliminated by Marcellinus, who by then was Magnentius' chief administrator.
The whole of the west, including Africa, now lay in Magnentius' hands. No challenge was made other than by the Germans on the Rhine, who acted as allies of Constantius II.
Seeking to further secure his position in case of an attack by Constantius II, Magnentius elevated his brother Flavius Magnus Decentius to the rank of Caesar (junior emperor).
But all the while the all-important legions on the Danube remained undecided. Eventually, rather than declare for Magnentius or Constantius II, they hailed their own emperor, Vetranio.
Magnentius made an effort to win Vetranio for his cause, but in vain, as it appeared Vetranio was a puppet of Constantius II who handed him control of the legions later that year and retired.
In AD 351 the war between Constantius II and Magnentius commenced.
With a large force raised in Gaul and Germany, which outnumbered the army of his opponent, Magnentius marched eastwards and defeated Constantius II at Atrans as he tried to invade Italy.
Constantius withdrew and Magnentius followed, leading to the bloodiest battle of the 4th century, the Battle of Mursa. Magnentius' army was utterly defeated, a total of 50'000 soldiers losing their lives.
Magnentius withdrew to Italy and sought to rebuild another army. But in the summer of AD 352 Constantius II invaded Italy, forcing Magnentius to flee to Gaul.
Having retreated to Lugdunum (Lyons), alas, with Constantius army in Italy and the barbarians crossing the Rhine, Magnentius realized his cause lost and committed suicide (AD 353).