As a native of Tuscany, Pope Leo was born around 400AD and served as a Deacon in Tuscany around the age of 31, but was extremely well known outside of Tuscany including in Rome. Emperor Valentinian III sent Leo (then Archdeacon) to help settle a dispute between one of Gaul's military commanders and the chief magistrate. This successful mission helped solidify confidence by the Imperial Court of his ability to lead. When Pope Sixtus III died in 440 AD, Leo was elected unanimously to succeed him.
During a time of strife in Rome (famines, poverty, warfare) Pope Leo distinctly pushed chartable works by Christians and associated the practices of fasting with charity. In an effort to maintain unity among the churches, he was one of the first Bishops of Rome to actively promote Papal primacy based upon direct succession from Saint Peter the Apostle. He consistently referred to himself the (unworthy) heir and deputy of Saint Peter, representing the authority and the Apostle but declaring consistently that Christ always was the source of all grace and authority.
Pope Leo presided over the Council of Chalcedon which rejected the the heresy of the Eutyches (denied the true human nature of the Son of God) and reaffirmed the union of both full human and full divine natures. In the official report, the following was written at the council:
"After the reading of the foregoing epistle, the most reverend bishops cried out: This is the faith of the fathers, this is the faith of the Apostles. So we all believe, thus the orthodox believe. Anathema to him who does not thus believe. Peter has spoken thus through Leo. So taught the Apostles. Piously and truly did Leo teach, so taught Cyril. Everlasting be the memory of Cyril. Leo and Cyril taught the same thing, anathema to him who does not so believe. This is the true faith. Those of us who are orthodox thus believe. This is the faith of the fathers. Why were not these things read at Ephesus? These are the things Dioscorus hid away."
When Attila invaded Italy in 452, Emperor Valentinian II sent three envoys to negotiate with Attila - Gennadius Avienus, Memmius Aemilius Trygetius, and Pope Leo. Though the specifics have been lost about what exactly happened during the negotiations, it is known that Attila quickly retreated afterwards and most ancient historians heaped praise on Pope Leo's actions during the negotiations, crediting him with the success of the mission.
Nearly 100 sermons and 150 letters written by Saint Pope Leo have been preserved, including his famous Tome. The Tome was a statement of faith of the Roman Church addressed to Archbishop of Flavian of Constantinople and was re-presented again at the Council of Chalcedon. In his In Nativitate Domini (Christian, Remember your Dignity) Pope Leo wrote of the obligations of living up to the common dignity in all Christians (Saints or Sinners):
Our Saviour, dearly-beloved, was born today: let us be glad. For there is no proper place for sadness, when we keep the birthday of the Life, which destroys the fear of mortality and brings to us the joy of promised eternity. No one is kept from sharing in this happiness. There is for all one common measure of joy, because as our Lord the destroyer of sin and death finds none free from charge, so is He come to free us all. Let the saint exult in that he draws near to victory. Let the sinner be glad in that he is invited to pardon. Let the gentile take courage in that he is called to life...
Let us put off then the old man with his deeds: and having obtained a share in the birth of Christ let us renounce the works of the flesh. Christian, acknowledge thy dignity, and becoming a partner in the Divine nature, refuse to return to the old baseness by degenerate conduct. Remember the Head and the Body of which thou art a member. Recollect that thou wert rescued from the power of darkness and brought out into God’s light and kingdom. By the mystery of Baptism thou wert made the temple of the Holy Ghost: do not put such a denizen to flight from thee by base acts, and subject thyself once more to the devil’s thraldom: because thy purchase money is the blood of Christ, because He shall judge thee in truth Who ransomed thee in mercy, who with the Father and the Holy Spirit reigns for ever and ever. Amen.
Pope Leo passed away on November 10th, 461 and was buried in Saint Peter's basilica, wishing to be buried as close as possible to the tomb of the Apostle. Pope Benedict XIV proclaimed him a Doctor of the Church in 1754.