Unlike the majority of the pretenders during the Crisis of the Third Century, Valerian was of a noble and traditional senatorial family. Details of his early life are elusive, but for his marriage to Egnatia Mariniana, who gave him two sons: later emperor Publius Licinius Egnatius Gallienus and Valerianus Minor.
In 238 he was princeps senatus, and Gordian I negotiated through him for Senatorial acknowledgement for his claim as emperor. In 251, when Decius revived the censorship with legislative and executive powers so extensive that it practically embraced the civil authority of the emperor, Valerian was chosen censor by the Senate, though he declined to accept the post. Under Decius he was nominated governor of the Rhine provinces of Noricum and Raetia and retained the confidence of his successor, Trebonianus Gallus, who asked him for reinforcements to quell the rebellion of Aemilianus in 253. Valerian headed south, but was too late: Gallus' own troops had killed him and joined Aemilianus before his arrival. The Raetian soldiers then proclaimed Valerian emperor and continued their march towards Rome. At the time of his arrival in September, Aemilianus' legions defected, killing him and proclaiming Valerian emperor. In Rome, the Senate quickly acknowledged him, not only for fear of reprisals, but also because he was one of their own.
Valerian's first act as emperor was to make his son Gallienus his colleague. In the beginning of his reign the affairs in Europe went from bad to worse and the whole West fell into disorder. In the East, Antioch had fallen into the hands of a Sassanid vassal, Armenia was occupied by Shapur I (Sapor). Valerian and Gallienus split the problems of the empire between the two, with the son taking the West and the father heading East to face the Persian threat.
By 257, Valerian had already recovered Antioch and returned the province of Syria to Roman control but in the following year, the Goths ravaged Asia Minor. Later in 259, he moved to Edessa, but an outbreak of plague killed a critical number of legionaries, weakening the Roman position. Valerian was then forced to seek terms with Shapur I. Sometime towards the end of 259, or at the beginning of 260, Valerian was defeated in the Battle of Edessa and taken prisoner by the Persians. Valerian's capture was a humiliating defeat for the Romans.
Gibbon, in The History of the Decline and Fall of the Roman Empire describes Valerian's fate :
The voice of history, which is often little more than the organ of hatred or flattery, reproaches Sapor with a proud abuse of the rights of conquest. We are told that Valerian, in chains, but invested with the Imperial purple, was exposed to the multitude, a constant spectacle of fallen greatness; and that whenever the Persian monarch mounted on horseback, he placed his foot on the neck of a Roman emperor. Notwithstanding all the remonstrances of his allies, who repeatedly advised him to remember the vicissitudes of fortune, to dread the returning power of Rome, and to make his illustrious captive the pledge of peace, not the object of insult, Sapor still remained inflexible. When Valerian sunk under the weight of shame and grief, his skin, stuffed with straw, and formed into the likeness of a human figure, was preserved for ages in the most celebrated temple of Persia; a more real monument of triumph, than the fancied trophies of brass and marble so often erected by Roman vanity. ' The tale is moral and pathetic, but the truth of it may very fairly be called in question. The letters still extant from the princes of the East to Sapor are manifest forgeries; ' nor is it natural to suppose that a jealous monarch should, even in the person of a rival, thus publicly degrade the majesty of kings. Whatever treatment the unfortunate Valerian might experience in Persia, it is at least certain that the only emperor of Rome who had ever fallen into the hands of the enemy, languished away his life in hopeless captivity.
In 258, by a new and absolutely merciless edict, bishops, priests, and deacons were executed immediately, men of senatorial and equestrian rank were punished with degradation and confiscation of goods to be followed by death if they refused to offer heathen sacrifice, women were threatened with confiscation of their property and exile, and Christians in the imperial household were sent in chains to perform forced labour on the imperial domains. In this persecution Christian Rome and Carthage lost their leaders: Pope Sixtus was seized on 6 August, 258, in one of the Catacombs and was put to death; Cyprian of Carthage suffered martyrdom on 14 September. Another celebrated martyr was the Roman deacon St. Lawrence. In Spain Bishop Fructuosus of Tarragona and his two deacons were put to death on 21 January, 259. There were also executions in the eastern provinces (Eusebius, VII, xii). Taken altogether, however, the repressions were limited to scattered spots and had no great success.
Some modern scholars believe that, contrary to Lactantius' account, Shapur I sent Valerian and some of his army to the city of Bishapur where they lived in relatively good condition. Shapur used the remaining soldiers in engineering and development plans. Band-e Kaisar (Caesar's dam) is one of the remnants of Roman engineering located near the ancient city of Susa. In all the stone carvings on Naghshe-Rostam, in Iran, Valerian is respected by holding hands with Shapur I, in sign of submission.
It is generally supposed that some of Lactantius' account is motivated by his desire to establish that persecutors of the Christians died fitting deaths; the story was repeated then and later by authors in the Roman Near East "fiercely hostile" to Persia.
Other modern scholars tend to give at least some credence to Lactantius' account.
Valerian and Gallienus' joint rule was threatened several times by usurpers. Despite several usurpation attempts, Gallienus secured the throne until his own assassination in 268.
Owing to imperfect and often contradictory sources, the chronology and details of this reign are very uncertain.