Leo was probably born in Rome of Tuscan parentage, but of his early life there is no record. Under Popes Celestine I and Sixtus III, he had at least the rank of deacon, sufficiently important for him to correspond with Cyril of Alexandria, and also to have a treatise of John Cassian on the Incarnation to be dedicated to him. Perhaps most significant was his being chosen by the emperor to settle the dispute between Aëtius and Albinus, the two highest officials in Gaul. Their feud which threatened the safety of Gaul from attack by the northern European tribes.

During Leo’s absence from Rome, Pope Sixtus III died on 11 August, 440, and Leo was unanimously elected by the people to succeed him. It was a post he would hold with great distinction for 20 years.

His term of office was significant for a number of reasons. His statement of the doctrine of the Incarnation of Christ, acclaimed at the Council of Chalcedon in 451, is regarded as a high point of Christian Church history. He also acted vigorously to free Rome from the power of the tribes invading from northern Europe, as well as restoring both the spiritual and material damage they caused.

Both his writings and his actions revealed a deep conviction that the doctrinal primacy of Rome was God-given and based on the scriptures. In fact, throughout his reign he strengthened and increased the influence and prestige of the papacy.

His 143 surviving letters reveal a similar concern for the Church in Spain, Gaul, and Africa. He also had to deal firmly with the resurgence of Manichaeism, Priscillianism and Pelagianism in different places. There were times when he had to reaffirm an ancient tradition by which a bishop had the right to appeal to Rome. For instance, he found that Hilary as an archbishop had exceeded his power over a fellow-bishop and overruled his decisions. He was always conscious of the power of Peter invested in him as Bishop of Rome.

In 452, Attila and Huns laid siege to Milan and terrorised the citizens by their violence. When Attila moved on against Rome, he was met by Leo who persuaded him to accept tribute rather than attack the city. Later, Leo tried the same methods in approaching the Vandal Genseric, but he was less cooperative. Even so, Leo’s influence was such as to make the Vandals stop short at looting the city and refrain from murdering its people and setting it on fire. However, while many prisoners were taken off to Africa, some church treasures were given back. Leo then sent priests and alms to the captives in Africa and in Sicily.

His greatest achievement was the acceptance of his dogmatic statement on the Incarnation of Christ at the Council of Chalcedon. He expressed with great clarity the doctrine that Jesus Christ is one Person, the Divine Word, in whom the two natures, human and divine, are permanently united without confusion or mixture. When the council heard this document read by Leo’s legates, they cried out:

Peter has spoken through Leo!

It would become, from then on, the official teaching of the Christian Church.

However, the same council raised the status of the see of Constantinople and, in spite of the legates’ protests, made it a patriarchate, mainly for political reasons. Here, as with the fall of Rome to the barbarians, Leo’s success was mixed with failure. He died on 10 November, 461 and was buried at St. Peter’s in Rome.

Leo’s personality has been described as one of indomitable energy, magnanimity, consistency, and devotion to duty. His principal writings to survive are 96 sermons and letters. He was declared a Doctor of the Church in 1754.