Almost nothing is known about the life of Saint Leo the Great before he became the Pope in 440 A.D. It is thought that he was born in either Tuscany or Rome around the year 400. He most likely came from an aristocratic family and he obviously had received a good education.
After Leo was ordained a deacon, he began working directly with Pope Saint Celestine I (unknown-432) and later with Pope Sixtus III (unknown-440). In 440, the latter sent Leo to settle a political dispute between two Roman generals named Albinus and Flavius Aetius.
Pope Sixtus III died while Leo was away on this mission. Upon the latter’s return, he discovered that he had been elected Pope. He was consecrated on September 29, 440.
The Church has, thankfully, been careful to preserve most of the writings from the papacy of Saint Leo the Great. 96 of his many sermons have been preserved and at least 143 of his letters. Because Leo lived during rather tumultuous times, he helped the world realize that the Pope not only has influence in spiritual matters, but also in political.
Throughout his papacy, Leo relentlessly fought against heresy. The main two he fought against were Nestorianism and Pelagianism. The Nestorians believed that there was some sort of barrier between the divine and human natures of Jesus Christ and that Mary was mother only of His human nature and, therefore, could not be called the Mother of God. The Pelagians believed that the sin of Adam did not entirely taint human nature. There were quite a few Pelagians in Rome and Pope Leo refused to let them receive Communion until they denounced their heretical beliefs.
In 452, Attila the Hun invaded Italy and ransacked Aquileia, Milan, and Pavia. Pope Leo, knowing that Rome was now threatened, went out personally to meet with him. There is a legend that, during their interview, Attila the Hun saw an enormous man, thought to be Saint Peter, who threatened to destroy the entire Hun army if they did not leave Italy. Whether this story is true or not, Pope Leo I has rightly been credited with preventing further destruction at the hands of the Huns.
Pope Leo I died of natural causes on November 10, 461. He was the first Pope to be given the posthumous title “the Great”. He has also been declared a Doctor of the Church and is sometimes considered an Early Church Father. His feast day is November 10.
Source: Butler’s Lives of the Saints