Hilarius was a politician of the Western Roman Empire

Western Roman Empire

The Western Roman Empire was the western half of the Roman Empire after its division by Diocletian in 285; the other half of the Roman Empire was the Eastern Roman Empire, commonly referred to today as the Byzantine Empire....

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Hilarius is known to be the praefectus urbi

Praefectus urbi

The praefectus urbanus or praefectus urbi, in English the urban prefect, was prefect of the city of Rome, and later also of Constantinople. The office originated under the Roman kings, continued during the Republic and Empire, and held high importance in late Antiquity...

of Rome in 408. He is attested in office on January 15 of that year by a law preserved in the Codex Theodosianus

Codex Theodosianus

The Codex Theodosianus was a compilation of the laws of the Roman Empire under the Christian emperors since 312. A commission was established by Theodosius II in 429 and the compilation was published in the eastern half of the Roman Empire in 438...

. He has been sometimes identified with a Hilarius Praetorian prefect of Gaul in 396, but this identification is rejected by historians on the fact that the urban prefecture was a lesser office than the praetorian one.