A Spanish Celt, he was born in about 335, and was proclaimed emperor while serving with the army in Britain. Having defeated his main opponent, Gratianus, he made his capital at Augusta Treverorum (Treves, Trier) in Gaul. A devout Christian, he was a stern persecutor of heretics.
According to the Mabinogion, Magnus Maximus (in Welsh, called "Macsen Wledig") took as his wife Helen or Elen, daughter of a chieftain based at Segontium (Caernarfon). Although the story of their meeting is undoubtedly fictional, there is some evidence for the basic facts. Welsh legend also claims that he was responsible for the withdrawal of Roman troops from Wales, twenty years before the rest of Britain was left to govern itself.
Magnus Maximus was defeated and killed in battle by Theodosius I, along with his son.