According to several inscriptions, Priscus was Praefectus of the province of Mesopotamia a frontier province at the border of Persia, therefore highly militarized. He was the Procurator of Macedonia, second in command to Egypt's governor and held judicial responsibilities in Alexandria. Priscus became a member of the Praetorian 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.