Before 421 he was sent to the powerful patricius Flavius Constantius (briefly Emperor in 421) to ask for a tax reduction for his own country; this embassy was successful.
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The new Emperor needed the support of both the civil institutions, the Roman senate and the Eastern Roman Emperor Marcian, as well as that of the army and its commanders (the generals Majorian and Ricimer) and the Vandals of Gaiseric.
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In the late spring of 455, Avitus was recalled to service by emperor Petronius Maximus and was elevated to the rank of magister militum, probably praesentalis; Maximus sent Avitus in an embassy to the court of Theodoric II, who had succeeded to his father, at Toulouse.
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Counting on the popular discontent, on the disbandment of the imperial guard, and on the prestige gained through their victories, Ricimer and the comes domesticorum Majorian rebelled against Avitus; the Emperor was obliged to leave Rome in early autumn and to move north.
After the deposition of Avitus, who ruled as emperor of the West from 455 to 456, there seems to have been a conspiracy among the Roman nobles to place the Pagan general Marcellinus on the throne to restore Paganism; but it came to nothing.
The historians Edward Gibbon and J. B. Bury believed that the acclamation took place at a regular meeting of the Concilium, but this has been rejected on the grounds that Avitus arrived in Arles at the wrong time of year for this, that the public meeting include representatives from outside the Seven Provinces and that Sidonius Apollinaris records that the meeting was arranged specifically to greet Avitus.