Priscus was born in the Syrian Roman province, possibly in Damascus, son of a Julius Marinus a local Roman citizen, possibly of some importance. The name of his mother is unknown, but his brother was Marcus Julius Philippus, later the Roman emperor known as Philip the Arab. Priscus was probably older than Philip, since the later political career was pushed by his own influence. According to several inscriptions, Priscus was prefect of the province of Mesopotamia, procurator of Macedonia, second in command to Egypt's governor and held judicial responsibilities in Alexandria. Priscus became a member of the Prateorian guard around 242 during Gordian III Persian campaign, and, when Timesitheus – the praetorian prefect – died in 243, he convinced the young emperor to substitute him with his own brother Philip. For a year, Priscus and Philip served as de facto regents of Gordian III.
When Gordian was killed by mutinous soldiers in 244, Philippus becomes the new emperor. As his brother and trusted ally, Priscus remains in the East, while Philip travelled to Rome. Priscus held supreme power in the Eastern provinces and his referred in inscriptions as rector Orientis. His rule was severe and oppressive. Following his brother's directives, Priscus collected heavy taxes that eventually led to rebellion and the uprising of Marcus Iotapianus, one of the four usurpers reported for Philip's reign. Apparently, Priscus managed to control this rebellion.
Following the defeat of his brother by the new emperor the usurper Decius, Priscus declared himself emperor in the Eastern provinces. The rebellion was not quickly controlled and Priscus had time to secure his position. In order to accomplish his purposes, He allied himself with the Goths that were attacking the Moesia province in the Danube frontier. Decius was killed, but Priscus rebellion remained unsuccessful. He was probably killed in the beginning of the reign of Trebonianus Gallus.