His birthdate is unclear, being his origins quite obscure, but it is widely believed that the late Roman Emperor, Petronius Maximus, belonged to the illustrious Anicius family. He developed his senatorial career during the reigns of Honorius and Valentinian III and was praetor in the first stage of his career in 411. Around 415, he served as tribunus et notarius, an entry position to the Imperial bureaucracy, which would take him to the position of comes sacrarum largitionum between 416 and 419, as well as urban prefect between 419 and 433.
In 433, he was elected consul, and from 439 to 441 held the praetorian prefecture of Italy. By the time he obtained the title of patrician in 445, he had become the Roman with most honors outside the Imperial circle.
According to John of Antioch, Valentinian III, under a plot instigated by Petronius Maximus, assassinated the powerful Roman general, Aetius, whom he believed was trying to usurp the throne. Petronius Maximus hated Valentinian because he had raped his wife and had asked for Aetius´ post, but the emperor rejected him, as counselled by Heraclius. To bring his vengeance to completion he used two accomplices, two Scythian mercenaries, who assassinated Valentinian and Heraclius in Campus Martius.
The death of Valentinian triggered a power vacuum in the already dying Western Empire. Several candidates to occupy the throne appeared: Maximian, Aetius´ comes domesticorum; Majorian, who had the support of Licinia Eudoxia (Valentinian´s widow); and Petronius Maximus, who managed to win the support of the Roman Senate and who finally secured the throne by bribing the officers of the Imperial Palace and forcing Valentinian’s widow, Aelia Eudoxia, to marry him.
In the court of Constantinople, Petronius Maximus´ accession to the throne was not officially recognized. To secure his position, Maximus named Avitus (who had great influence among the Visigoths) as his magister militum and sent him to Tolosa, the Visigothic court, to win their favour. Nevertheless, Maximus had another powerful enemy to contend with: King Genseric of the Vandals. Genseric considered that his truce with Rome was valid only with Valentinian, and since Valentinian was dead, he was no longer bound to it. Also, Maximus had cancelled the betrothal between Eudocia (Licinia´s daughter) and Huneric (Genseric´s son) in order to marry Eudocia to his own son, a way, he thought, of ensuring his lineage. By May 455, two months after Maximus´ accession to the Roman throne, Vandals set sail to Italy.
As the news spread, panic overtook the citizens of Rome. Knowing he was not strong enough to confront the Vandal army, Maximus planned his flight with the Senate. During the turmoil, he was abandoned by his bodyguards, and in May 31, while riding out of the city, he was intercepted by an angry mob and stoned to death. According to other sources, he was assassinated by a soldier called Ursus and his body mutilated and flung into the River Tiber. Palladius, son from his first wife, who had been appointed Caesar, was also probably killed in the mayhem.
Three days after the death of Petronius Maximus, Genseric’s Vandal army entered Rome and sacked the city. Rome’s total destruction was avoided, however, thanks to the pleas of Pope Leo I.