He was born in Cyrrhus, and was the son of Gaius Avidius Heliodorus, who served as praefectus or governor of Roman Egypt, and Julia Cassia Alexandra, who was related to a number of royal figures, including her descent from both Augustus and Herod the Great.
He served during the Parthian war of Lucius Verus, in which he distinguished himself, for which he was elevated to the Senate, and later made Imperial legate.
During the Bucolic War, he was given the extraordinary title of Rector Orientis, giving him Imperium over all of the eastern provinces of the Roman Empire.
[5] Cassius was also a distant descendant of the Roman client-king Antiochus IV Epiphanes of Commagene, who had been dethroned half a century before.
Despite the fact that Seleucia had surrendered to the Romans, he destroyed it as well, justifying it by claiming that the local population had broken their agreement.
Much of his success was credited to Emperor Lucius Verus, who, although himself an excellent commander, was notedly unafraid of delegating military tasks to more competent generals.
[15] During that year, Lucius Verus and Cassius launched a new campaign against the Parthians, invading across the northern stretch of the Tigris river, into Media.
During this time, a false rumor reached Rome that Cassius had led Legio III Gallica across the Indus River.
[17] In c.170, Cassius was given the extraordinary title of Rector Orientis (literally "Supreme Commander of the Orient"), giving him imperium over the entirety of the eastern section of the empire,[18] in order to combat a large rebellion in Egypt, commonly called the Bucolic War.
[22] The Historia Augusta, a work both treasured by historians and notorious for its falsehoods, records that Aurelius formed a peace commission among his advisors.
[4] Despite controlling some of the most important parts of the Roman East, especially Egypt which was a critical supplier of grain for the city of Rome, Cassius failed to win widespread support for his rebellion.
[28] Many nobles throughout the empire opposed the rebellion, one example being Herodes Atticus, who is recorded as having sent Cassius a letter containing only the word emanes, literally "you are mad".
Despite this widespread opposition, the capital of Rome was thrown into a panic, which forced Aurelius to send Gaius Vettius Sabinianus Julius Hospes, the governor of Pannonia Inferior, with troops to secure the city.
Avidia Alexandra, the daughter of Cassius, and her husband, were placed under the protection of "an uncle by marriage", believed to be Claudius Titianus, a Lycian senator.
[34] Dio spoke highly of Cassius, saying that he was a "good man" whose only fault was that his father, Heliodorus, was given his post of Praefectus augustalis only because of his speaking abilities.