Nothing is known of Justin's origins or early life. He appears for the first time in 528, when along with Narses he was sent to Italy with 7,000 men as reinforcements for Belisarius, who had just successfully survived siege of Rome by the Ostrogoths. At the time, he held the position of "magister militum per Illyricum", a post he may have been appointed to already in 536, after the death of general Mundus. In the dissension that broke out in the Byzantine army between Belisarius and Narses, Justin sided with the latter, and accompanied him to the relief of the Gothic siege of Ariminum, defended by the general John. After the successful outcome of the operation, along with John, Justin proceeded to occupy the region of Aemilia against little Gothic resistance during the winter of 538/539. The rift in the imperial army had by this time deepened to the point that Justin and John outright refused to obey orders from Belisarius to march to the aid of the city of Mediolanum, which was being besieged by the Goths with their Frankish allies, instead waiting for relevant orders from Narses. The delay proved fatal, and the great city was captured and razed by the Franko-Gothic army.