Hilarius (Latin Hilarius, 'cheerful') was a Sardinian by birth who had served Pope Leo I as his principal personal deacon and legate for over a decade before succeeding Leo as Bishop of Rome on November 19, 461. Hilarius had been Leo's personal representative at the catastrophic 449 Robber Council of Ephesus (which had reversed the Chalcedonian Christological line and at which Hilarius had to escape secretly to Rome to inform Leo); the experience formed his subsequent pontificate.
His six-and-a-half-year pontificate was principally devoted to two areas. First, the substantial reorganization of the Catholic episcopal structure of southern Gaul: Hilarius held a Roman synod in 465 (the first held in the Lateran Basilica in Rome since the foundational anti-Donatist synod of 313) that addressed irregularities in episcopal elections in Gaul and reasserted the foundational Roman-papal Gallic supervisory role.
Second, the substantial architectural-liturgical expansion of the Lateran Baptistery complex at Rome: Hilarius built the famous three small devotional-chapel oratories adjacent to the central Lateran Baptistery (the Oratory of St. John the Baptist, the Oratory of St. John the Evangelist, and the Oratory of the Holy Cross — the latter the only one to survive substantially intact). The three chapels are the foundational example of the Roman-papal devotional-chapel architectural form that would become standard in the post-patristic Latin tradition. The Oratory of the Holy Cross was constructed specifically to house the relic of the True Cross that had been recently translated to Rome.
He died on February 29, 468 (a leap-year date, accounting for the difficulty of the calendar-placement of his feast). He was buried at the Lateran Baptistery in the small oratory complex he had built.