From "The Oxford Classical Dictionary":

Magnentius, Flavius Magnus, from a family of barbarian settlers in Gaul, rose to a senior military command under the emperor Constans. In January AD 350 at Autun (Augustodunum) he led a coup which overthrew Constans, and rapidly won over the western provinces; although nominally a Christian, he made religious concessions to the pagan senatorial aristocracy. He failed to gain the recognition from the eastern Augustus Constantius II, and his forces were defeated by those of Constantius at the epic battle of Mursa in 351. His resistance finally ended with his suicide in Gaul two years later.

From "The Roman Emperors" by Michael Grant:

“Magnentius (Flavius Magnus) (breakaway emperor in the west, 350-3) was born c. 303, in or near Ambiani (Samarobriva), reportedly of a British father and a Frankish mother. His wife (later married to Valentinian I) was Justina. Magnentius served in a barbarian contingent in the reign of Constantine the Great, becoming a staff officer (protector) and then a field army commander (comes rei militaris) under his sons, who put him in charge of the elite legions known as the Joviani and Herculiani.’

“On 18 January 350, Marcellinus, finiancial minister of the emperor Constans I, held a birthday party for his sons at Augustodunum, at which Magnentius appeared in purple robes and was acclaimed as Augustus. The imperial army deserted to his cause, and Constans I, fleeing towards Spain, was killed by one of his adherents. In June Nepotianus, the son of Constantine the Great’s sister Eutropia, refused to accept Magnentius’ rule and declared himself emperor, defeating a levy raised by the praetorian prefect Anicetus, but after twenty-eight days he and his mother, and other members of his family, were killed by Marcellinus, who had by then become Magnentius’ chief administrative controller.’

“The whole of the west, including Africa, then recognized Magnentius, whose coins honour him as ‘Restorer of Liberty’ (RESTITVTOR LIBERTATIS), to point a contrast with the unpopular regime of Constans I. On the Danube, however, another usurper, Vetranio, after first supporting his cause, went over to Constantius II, who in the meantime prompted the Germans on the Rhine to harass Magnentius; while he for his part, believing an attack from Constantius himself to be imminent, nominated Flavius Magnentius Decentius, who was his relative and probably his brother, as Caesar.’

“Meanwhile an exchange of envoys had been taking place. Already in 350, while Vetranio was still Augustus, Magnentius sent a senator named Nunechius (perhaps his governor of Gaul) and his principa commander (magister militum) Marcellinus – not the same man as the chief administrative officer (magister officiorum) of that name – to Constantius, who placed them under arrest. Then the other Marcellinus met Constantius’ representative Flavius Philippus and escorted him to Magnentius. Philippus’ purpose was ostensibly to negotiate a peace settlement with the pretender, but his real objective was to find out his military dispositions on Constantius’ behalf. Addressing Magnentius’ army, he rebuked them for disloyalty to the house of Constantine and proposing that Magnentius should limit his territories to Gaul. His oratory caused the troops to waver, but Magnentius, after reminding them that it was the oppressive government of the imperial house that had prompted them to revolt in the first place, refused to allow Philippus to return to Constantius and placed him under arrest.’

“In 351 the inevitable hostilities between the two rivals broke out. Magnentius had raised large forces in Gaul, including numerous Germans, and in consequence outnumbered Constantius II, who as he advanced westwards was defeated with heavy losses at Atrans, on the border of Italy and Noricum, and compelled to retreat. Rejecting his offer of a compromise, Magnentius left his headquarters at Aquileia to march into the Danubian provinces. There, despite an initial lack of progress, he established himself at Mursa Major in the rear of Constantius’ army, thus forcing him to fight backl but after a prolonged engagement Magnentius’ right wing was routed by the opposing cavalry and he suffered a total defeat – the first reverse, it would appear, that heavy cavalry had ever inflicted on legionaries. Magnentius reportedly lost twenty-four thousand men and Constantius thirty thousand; this was the bloodiest battle of the century, and inflicted irreparable losses upon the Empire’s military strength. Magnentius recoiled to Aquileia and endeavoured to rebuild his army. But in the summer of 352, unable to withstand Constantius II’s invasion of Italy, he was obliged to withdraw to Gaul. There in the following year he was defeated again, and lost control of the Rhine frontier (which was temporarily overrun by barbarians). Next he retreated precipitately to Lugdunum, where, seeing that his position was hopeless, he committed suicide, leaving the entire Roman Empire in Constantius’ hands.’

“Magnentius had made himself unpopular with the upper classes by rigorous taxation, so that the literary authorities offer an unfavourable view of his regime, ignoring whatever military, administrative or diplomatic gifts he may have possessed. He was a pagan and favoured the pagans, but was forced by political necessity to set this preference aside owing to the need to rally the orthodox Catholic (Nicene) party to his cause against the Arians who supported Constantius. These endeavours were displayed by a remarkable coinage of 353 which, for the first time in the history of Rome, devoted the principal part of the design to a Christian emblem, the Chi-Rho sign (XP in a monogram, standing for Christos) flanked by Alpha and Omega and inscribed ‘The Well-Being of our Lords the Augustus and the Caesar’ (Decentius) (SALVS DD. NN.AVG.ET CAES. [Dominorum Nostrorum Augustii at Caesaris]). These issues were made at the mint of Ambiani (Samarobriva). Magnentius and Decentius had also coined at Rome, Aquileia and Treviri; and after the loss of Treviri, towards the end of his reign, Lugdunum and Arelate became mints instead.’