His contemporary historians were biased against him, particularly Procopius and John the Lydian, and their accounts are often coloured by their prejudices. Procopius calls John poorly educated, though he grudgingly admits the Cappadocian's great natural abilities led to his rise to prominence. He first appears in the sources as a "scriniarius" (notary) in the service of an unidentified "magister militum". His administrative ideas reportedly attracted the attention of Emperor Justinian, resulting in his promotion to positions with financial responsibilities. From there, he gained enough favour to become a "vir illustris" and eventually the acting Prefect. There is a theory that his close relationship with Emperor Justinian may date to Justinian's service as "magister militum praesentalis" in the 520s, prior to his elevation to the Byzantine throne.