Petronius Maximus was born in about AD 396, his birthplace being unknown.
Being of obscure origin, Petronius Maximus achieved remarkable career early on in his life.
After this he became praetorian prefect for Italy two, perhaps even three times and was twice city prefect of Rome and twice consul. His career in AD 445 earned him promotion to the rank of patrician (patricius).
With the murder of Valentinian III in AD 455 there was no heir to the western throne.
However, it was Petronius Maximus who eventually was chosen. True, he had great experience in administration, through having held high offices earlier. And yet, it is largely believed that he used his extensive wealth to buy himself favour and hence literally bought himself the throne.
On taking up office as emperor, Maximus immediately married Licinia Eudoxia, the widow of Valentinian III. She only married him reluctantly, suspecting that he in fact had been involved in the murder of her late husband.
Despairing, Licinia Eudoxia eventually appealed for help to the Vandal king Geiseric. Licinia Eudoxia of course already had contacts to the Vandal court as her daughter Eudocia had been betrothed to Geiseric's son Huneric - before Petronius Maximus had cancelled the arrangement.
By May news reached Rome that Geiseric was sailing for Italy.
Though in the panic Petronius Maximus was completely abandoned, left to fend for himself by his bodyguard and entourage.
On 2 June AD 455 Geiseric captured the city of Rome and thoroughly sacked it for two weeks. He left, carrying away a great amount of loot as well as the empress Licinia Eudoxia and her daughters Placidia and Eudocia. (Eudocia married Huneric in AD 456 as had been originally intended.)