Petronius Maximus was born in about 396.

Although he was of obscure origin, it is now believed that he belonged to the Anicii family.

Related to later Emperor Olybrius, Maximus was the son of Anicius Probinus,,the son of Anicia Faltonia Proba and Sextus Claudius Petronius Probus, who was Prefect of Illyricum in 364, Prefect of Gaul in 366, Prefect of Italy in 368–375 and again in 383 and consul in 371.

Maximus had achieved a remarkable career early on in his life.

His earliest known office was praetor, held about 411; around 415 he served as a tribunus et notarius, which was an entry position to the imperial bureaucracy, and led to his serving as Comes sacrarum largitionum (Count of the Sacred Largess) between 416 and 419.

From January/February 420 to August/September 421 he was praefectus urbi of Rome, an office he held a second time sometime before 439; as praefectus, he restored the Old St. Peter's Basilica.

He was also appointed praetorian prefect sometime between 421 and 439; it was either during his holding of this post, or during his second urban prefecture, that he was appointed consul for the year 433.

From August 439 to February 441 he held the praetorian prefecture of Italy, then a second consulship in 443.

Between 443 (the year of his fourth prefecture and second consulship) and 445 (the year he is granted the title of Patrician) Maximus has built a forum in Rome, on the Caelian Hill between via Labicana and the Basilica di San Clemente.

During this year, he is briefly the most honored of all non-Imperial Romans, until the third consulate of Aëtius, generalissimo of the Western empire, the following year.