Anyway, Petronius Maximus entered the Senate through the position of praetor c. 411 and continued to have a most illustrious senatorial career. He served as urban prefect of Rome on several occasions, leader of the Roman nobility, Praetorian prefect of Italy, and twice served as consul (433 & 443). In 445 he attained the rank of patrician. By this time, he was the leading noble of Rome.
It has been suggested, and seems probable, that he was behind the assassination of the Emperor Valentinian III in 455, and it is certain that he befriended those responsible for this murder. Upon the removal of the Emperor, Maximus usurped his position making himself Western Roman Emperor with the support of the Senate. In a move designed to shore up his legitimacy, he forcibly married Valentinian III's widow, Licinia Eudoxia. Likewise, he named his son Palladius to the position of Caesar and had him marry one of Valentinian's daughters, most likely, Eudoxia.
Within the few months of his reign, he appears to have had a degree of success in getting himself recognized as Emperor in the West, though he was not recognized by the Eastern Emperor Marcian. Soon, however, he learned that that the Vandal King of Africa, Gaiseric was approaching the city. Though the exact circumstances of the panic are unknown, it is known that Maximus did indeed panic, giving leave to those who wished to flee the city and planning to do so himself as well. With the arrival of the Vandals, Maximus tried to flee the city, but was captured by his fellow Romans, killed, and tossed without ceremony into the Tiber. It would appear that many believe that the Vandal invasion was a direct result of Maximus' forcible marriage to Licinia Eudoxia though this appears to be very unlikely.
Some of the secondary sources I have reviewed seem to suggest that Petronius Maximus was not only a member of, but the leader of, the Anicii in Rome during this time. This can possibly be explained by one of three ways. Firstly, this would be true if Petronius Maximus was born into the gens Anicii, but as of yet, I have found no evidence to this effect. Secondly, it could be explained if Maximus married into the Anicii, but again, I have nothing suggesting this. It is known that Petronius Maximus was married before his forcible marriage to Licinia Eudoxia and that by this marriage his son, Palladius, was born. However, as the Anician name was universally recognized as the very apex of the Roman nobility, virtually everyone who married into the Anicii opted to give their children Anician names, and Pallidius is, most assuredly, not an Anician name.
The third, and at present most likely, explaination regards Anician support of Maximus. At this time, the Anicii were still the richest and most august family in the city of Rome which means that they inevitably still held a great deal of political clout. It would seem unlikely that Maximus, a man of undistinguished or at least obscure origins, would have advanced as high as he did without the support of the Roman Anicii. In this sense, Maximus, could have been the de facto leader of the Anicii, without actually being a member of the family. In support of this, the Cambridge Medieval History says,
"The assassination of Valentinian himself was followed by the accession of Maximus, a member of the great senatorial family Anicii; and it has been suggested that the accession of Maximus perhaps indicates an attempt of the Anicii to establish a new government in the West, independent of Constantinople and resting on the support of the Senate."[vol.1, p.397]
And later on the same book says,
"The assassination of Valentinian III had been followed by the accession of Maximus. The head of the great family of the Anicii, Maximus was the leader of the Senatorial and Roman party; and his accession would seem to indicate an attempt by that party to institute a new government, independent at once of the magister militiae at home and the Eastern Emperor at Constantinople. But it was an age of force and in such an age such a government had no root."[Vol.1, p.420-421]
The correlation between the "Senatorial and Roman Party" and the Anicii appears quite clear here, as does the suggestion that Maximus was the head of this party. From this perspective, Maximus may very well have been leader of the Anicii, though not actually related to the family.
Anyway, for an excellent online biography of Petronius Maximus, visit the DIR site.