Born in Sardinia, Saint Hilary first served as an archdeacon under Pope Leo I where he fought for the rights of the Roman See. He became known for his devout faith and staunch defense of these rights so much that he served along side Bishop Julius of Puteoli in 449 as papal legates to the Second Council of Ephesus. It was at this council, known as the robber council, that the two had arrived with a very critical letter from Pope Leo. The notary, working with the Emperor, declared that the emperor's letter be read first and due to the chaotic nature of the council, Pope Leo's letter was never given the opportunity to be read. Saint Hilary staunchly opposed the condemnation of Saint Flavian at the council and annulled the sentence in Pope Leo's Name.
Immediately he received the ire of Pope Dioscorus I of Alexandria for these actions at the council. Diocurus tried to prevent Saint Hilary from leaving Ephesus and hindered every attempt at his travels to either Rome or Constantinople. Eventually though, Saint Hilary was able to make it home to Rome where he presented news of the events of the council and Saint Flavian's death. Saint Hilary attributed his escape from the council to the intercession of Saint John the Apostle. In a letter he wrote to Pope Leo I, he apologized for not being able to deliver the Pope's letter at the synod.
On November 19th, 461, Saint Hilary was elected Pope. Initially he worked tirelessly to reconfirm the supremacy of the Bishop of Rome and to strengthen the ecclesiastical government in Spain. The now Pope Hilary also countered the emperor's edict of toleration for schismatic sects in 467 and openly rebuked the emperor on his visit to Saint Peter's Basilica for the acts of a member of Emperor Anthemius's court, Philotheus. Philotheus had been strongly supporting the Macedonian heresy. Throughout the rest of his Papacy, Saint Hilary would find himself involved in numerous ecclesiastical disputes throughout the European continent. He oversaw the Roman synod of November 19, 465 AD that was held in the Basilica of Santa Maria Maggiore where the Bishop of Irenaeus was told to quit the see of Barcelona and return to his former see. This synod is the oldest Roman synod where the original records have survived to the modern day.
Also during his time as Pope Saint Hilary completed several construction projects around Rome including a chapel of the Holy Cross in the baptistery, several convents, public baths and libraries. Two oratories were built in the baptistery of the Lateran - one for Saint John the Baptist and the other for Saint John the Apostle.
Saint Pope Hilary died on February 29th, 468. His feast day is celebrated on February 28th on non leap years and the 29th on leap years.