The place and date of birth is unknown- died 10 November, 461. Leos pontificate, next to that of St. Gregory I, is the most significant and important in Christian antiquity. At a time when the Church was experiencing the greatest obstacles to her progress in consequence of the hastening disintegration of the Western Empire, while the Orient was profoundly agitated over dogmatic controversies, this great pope, with far-seeing sagacity and powerful hand, guided the destiny of the Roman and Universal Church.
According to the Liber Pontificalis (ed. Mommsen, I, 101 sqq., ed. Duchesne, I, 238 sqq.), Leo was a native of Tuscany and his fathers name was Quintianus. Our earliest certain historical information about Leo reveals him a deacon of the Roman Church under Pope Celestine I (422-32). Even during this period he was known outside of Rome, and had some relations with Gaul, since Cassianus in 430 or 431 wrote at Leos suggestion his work De Incarnatione Domini contra Nestorium (Migne, P.L., L, 9 sqq.), prefacing it with a letter of dedication to Leo.
About this time Cyril of Alexandria appealed to Rome against the pretensions of Bishop Juvenal of Jerusalem. From an assertion of Leos in a letter of later date (ep. cxvi, ed. Ballerini, I, 1212- II, 1528), it is not very clear whether Cyril wrote to him in the capacity of Roman deacon, or to Pope Celestine. During the pontificate of Sixtus III (422-40), Leo was sent to Gaul by Emperor Valentinian III to settle a dispute and bring about a reconciliation between Aëtius, the chief military commander of the province, and the chief magistrate, Albinus.
This commission is a proof of the great confidence placed in the clever and able deacon by the Imperial Court. Sixtus III died on 19 August, 440, while Leo was in Gaul, and the latter was chosen his successor. Returning to Rome, Leo was consecrated on 29 September of the same year, and governed the Roman Church for the next twenty-one years.