Quietus was the son of Fulvius Macrianus and a noblewoman, possibly named Iunia. According to Historia Augusta, he was a military tribune under Valerian, but this information is challenged by historians.
He gained the imperial office with his brother Macrianus Minor, after the death of Emperor Valerian in the Sassanid campaign of 260. With the army deep in the enemy territory, the soldiers elected the two emperors. The support of his father, controller of the imperial treasure, and the influence of Balista, Praetorian prefect of the late Emperor Valerian, proved instrumental in his promotion.
Quietus and Macrianus, elected consuls, had to face the lawful Emperor Gallienus, at the time in the West. Quietus and Ballista stayed in the eastern provinces, while his brother and father marched their army to Europe to seize control of the Roman Empire. After the defeat of his brother and father in Thrace in 261, Quietus lost the control of the provinces in favour of Odaenathus of Palmyra. Forced to flee to the city of Emesa, he was killed by its inhabitants, possibly instigated by Ballista (Zonaras xii.24).