By AD 373, Antonius was serving as the quaestor sacri palatii, where his tasks included drafting imperial speeches for Valentinian for addressing the Senate.
[4] The death of Valentinian in 375 saw a purge of the bureaucracy by Gratian, but Antonius not only survived, but managed to prosper under the new administration.
During this period Antonius was a supporter of Ausonius, and was a key mover in the creation of a court alliance between the Gallo-Roman and Romano-Spanish aristocracy, which involved the rise of the general and future emperor, Theodosius.
[6] Antonius was responsible for implementing Gratian's school law of May 376 (subsidising the employment of grammarians and rhetoricians), as well as edicts which clarified the relationship between the civil and military judicial spheres in the prefecture of Gaul and solidifying the authority of the diocesan vicarius.
[7] After his tenure as praetorian prefect, Antonius followed the emperor Theodosius I and moved himself to the eastern court at Constantinople, and in AD 383, he was appointed consul prior, with Afranius Syagrius as his colleague.