Pope Saint Leo III, whose exact birth date remains a mystery, ascended to the papacy in dramatic fashion on December 26, 795—the very day his predecessor was laid to rest. His swift consecration the next day suggests that the Roman clergy were eager to secure their choice before external powers, particularly the Franks, could exert influence over the election. A native of Rome, Leo was the son of Atyuppius and Elizabeth. At the time, he held the title of Cardinal-Priest of the church of St. Susanna and possibly served as vestiarius, overseeing the papal treasury and vestments.
Upon his election, Leo sent Charlemagne not only news of his unanimous selection but also the keys to the Confession of St. Peter and the city's banner—powerful symbols of loyalty and acknowledgment of Charlemagne as the defender of the Holy See. In return, Charlemagne responded with commendatory letters and a share of the wealth seized from the defeated Avars. These resources allowed Leo to become a generous patron of Rome’s churches and charities.
Yet Leo's reign was quickly marred by violence and treachery. Driven by ambition and resentment, relatives of the late Pope Adrian I conspired against him. On April 25, 799, as Leo led the procession of the Greater Litanies toward the Flaminian Gate, he was assaulted by a gang of armed men. They threw him to the ground and attempted to gouge out his eyes and cut out his tongue. Left bloodied and broken, he was transported to the monastery of St. Erasmus on the Cœlian Hill. Miraculously, Leo recovered his speech and vision, an event many saw as divine intervention. Escaping captivity, he sought refuge with Charlemagne in Paderborn, accompanied by Roman supporters.
Despite attempts by his enemies to poison the emperor’s opinion, Leo was received with great honor. Charlemagne eventually had him escorted back to Rome, where he was celebrated with overwhelming public support. The conspirators were tried by imperial envoys, and unable to prove their case, were sent into exile.
Charlemagne himself visited Rome in 800, where the pope’s accusers were summoned again. Though the assembled bishops claimed no authority to judge the pope, Leo voluntarily took an oath proclaiming his innocence. In an act of mercy, he requested exile instead of execution for his primary enemies.
The most momentous event of Leo’s papacy occurred shortly after. On Christmas Day, 800, as Charlemagne knelt before the tomb of St. Peter, Leo placed a crown upon his head, proclaiming him “Emperor of the Romans.” This coronation marked the revival of the Western Roman Empire and affirmed the papacy’s vision of a unified Christian world under one spiritual and one temporal ruler—Christ's vicar on earth and the emperor.
Leo even supported a strategic proposal for Charlemagne to marry Empress Irene of Byzantium, a union that might have fused East and West. However, Irene’s ousting in 801 derailed that ambition.
In 804, Leo crossed the Alps to meet Charlemagne once more, possibly to assist in the division of the emperor’s lands among his sons. He later endorsed the final arrangements. Their alliance continued to impact broader Christendom: Leo and Charlemagne jointly addressed the Spanish heresy of Adoptionism and reorganized ecclesiastical jurisdictions in Central Europe.
Leo also intervened independently in English church matters. He condemned lay control of monastic communities and excommunicated Eadbert Praen for unlawfully seizing Kent’s throne. At the urging of Canterbury’s archbishop, he revoked the pallium granted to Lichfield, reasserting Canterbury’s primacy in line with Pope Gregory the Great’s earlier arrangement.
He was called upon to mediate disputes between Archbishop Wulfred of Canterbury and King Cenulf of Mercia—a conflict that saw the archbishop suspended and the kingdom under ecclesiastical sanctions for six years. Financial greed appeared to drive Cenulf’s actions, including his harsh treatment of the monastery at Abingdon, which only ceased after a hefty bribe and papal persuasion.
Meanwhile, in the East, the Church of Constantinople faced turmoil. Monks led by figures like St. Theodore the Studite condemned Emperor Constantine VI for divorcing his wife to marry Theodota. Though Patriarch Tarasius disapproved, he refrained from excommunication, angering the monks. Persecuted and exiled, these monastics appealed to Leo, who responded with gifts and encouragement. After Michael I took the throne, Leo ratified a peace treaty between East and West, further cementing his role as a diplomatic bridge-builder.
Throughout his papacy, Leo remained closely aligned with Charlemagne, even coordinating naval defenses against Saracen raids. Recognizing his own limitations, Leo entrusted the defense of Corsica to the emperor. He also reclaimed Church lands near Gaeta and ensured their administration under papal authority.
However, Charlemagne’s death in 814 marked a turning point. Another plot against Leo emerged but was thwarted in time. The conspirators were arrested and executed. Soon after, noble unrest in the Campagna escalated into rebellion, which was suppressed by the Duke of Spoleto under the King of Italy's command.
Despite these tribulations, Leo channeled his resources—bolstered by Charlemagne’s generosity—into church restoration and charitable works. He enriched Roman and even Ravennate churches with artistic embellishments, especially mosaics. His titular church, St. Susanna, once bore his likeness in shimmering tesserae until the 16th century.
Pope Leo III died on June 12, 816, and was interred in St. Peter’s Basilica alongside other Popes named Leo. Canonized in 1673, he remains a towering figure of medieval Christendom. Surviving silver denarii inscribed with both his name and Charlemagne’s testify to their powerful alliance—an enduring symbol of the emperor’s role as guardian of the Church and sovereign of Rome.