He is supposed to have been the second bishop of Aquileia, succeeding Hermagoras. During the persecution of Numerian he was tortured to death under the prefect Beronius. Before his death Hilarius's prayers brought about the collapse of the pagan temples in Aquileia and the images of the gods they contained, to which he had refused to sacrifice.
His martyrdom was shared by his deaconTatianus, otherwise Tatian, with whom Hilarius's name is often linked, as in the dedication of Gorizia Cathedral, and also by their companions Felix, Largus and Dionysius.
The feast day of Hilarius and companions is 16 March, the date of their martyrdom.