Birth. (No date found), Rome. Of a modest family. Son of Azupio (or Atzuppio). The name of the father made some sources think that the family had an eastern origin.

Education. In his childhood, he was reared in the vestiario of the Roman Church, the office that administered the papal treasury.

Early life. In his youth, he entered the service of the papal curia. He was ordained deacon and then priest.

Cardinalate. Presbyter cardinalis of the title of S. Susanna at an unknown date. He retained an important role in the papal administration and was named vestiario in 789, thus becoming one of the main collaborators of Pope Adrian I. In that capacity, he restored and embelished some Roman churches. He demonstrated great zeal providing wealth for the office of vestiario, soliciting alms and legacies from the faithful, ostensibly destining them to assist the poor.

Papacy. Elected pope by the clergy, lay nobility and people of Rome on December 26, 795, the same date of his predecessor's death. Took the name Leo III. Consecrated, Sunday December 27, 795. The election was bitterly contested by a group of troublemakers, especially the relatives of the deceased pontiff, who occupied important offices in the city, and the Roman duchy, who incited hatred against the new pope because he would not favor them as Adrian I had done. Shortly after his election, the new pope sent Charlemagne a letter of communication not to get the approval of the appointment but to bear witness of loyalty and obedience, as indicated by the gifts sent to the monarch: the keys of the tomb of St. Peter and the banner of the city of Rome. Those gifts meant that the pope recognized in him the proper authority of the defensor of the Christian religion with the use of arms, symbolized in the flag, but also the custodian of the tomb of the altar of the Confession to which Charles in 774 had made an oath with a "promise of donations." It seems clear that Charlemagne was convinced that the real influence of the pope should be confined to being the most important metropolitan of the Church, but that it was Charlemagne who, given the religious concept of his high office, was to undertake the task of propagating and defending the Christian faith. It was Charlemagne who assigned to Pope Leo III to provide for the organization of the Church among the Avars, which he had subdued; but the center of that mission was Salzburg, because that was what Charlemagne wanted. The monarch's interference in church affairs had started at the time of the second Council of Nicea (787), which was held without having been requested by the Church, and it was Charlemagne who told the pope to convene a council at Rome in 798, to condemn again the doctrine of Adoptionism (1), which Bishop Félix of Urgel continued to defend, despite that the heresy had been banned by the council of Frankfurt in 794. In September 797, the pope sent the pallium to Archbishop Eanbald II of York. In Rome, the nobles related to the previous Pope Adrian I resented that the new pontiff showed such a deferential attitude toward Charlemagne, which they saw as an indication of a weak temperament. Primicerius Pasquale and Scellario Campulo sought to profit from the ambitions of the iudices, and began to circulate charges of perjury and lasciviousness against Pope Leo III. Some rumors reached the court of Charlemagne, but Archbishop Arno of Salzburg assured Alcuin, the trusted ecclesiastical adviser of the king, that they were slander.

Believing that the king was not going to take any action against the pope, Pasquale and Campulo thought of devising a real conspiracy, which involved the removal of Pope Leo III and the conquest of power. The opportunity presented itself during the traditional procession made by the pontiff on April 25, 799, for the feast of S. Marco, from the Lateran to S. Lorenzo in Lucina. The pope proceeded on horseback and the conspirators attacked him at the convent of S. Silvestro, they pushed him off the horse and tried to blind him and tore his tongue, but the uproar was enormous, and only two managed to drag him into the cloister, where he was entrusted to Greek priests (sign that part of the clergy -the philobyzantine- agreed with the conspirators). That night, the pope was transferred to S. Erasmo al Celio, but from there, he was able to escape with the help of some of his loyalists, including Camerario Albino,who let the pope descend with a rope along the wall and took him battered but save to St. Peter. Much of the clergy and the people at this point were pressed around the basilica and the conspirators were unable to enter; the latter vented their anger plundering the houses of Albino and the relatives of the pope. Then there was the intervention Duke Vinigi of Spoleto, who happened to be in Rome; the day after, he led the pope to security in his city. The news of aggression, meanwhile, had reached Charlemagne, who was engaged in Paderborn with the Saxons; Pope Leo III wanted to go to him. The king, when he learned that the pope was seeking shelter under his protection, sent an embassy with the archbishop of Cologne Ildibad, and his son Pepin. The monarch received the pope with the highest honors; at the same time, came from Rome a memorial of the conspirators outlining the substance of their allegations, and asking to judge the behavior of the pope. Charlemagne turned to his faithful Alcuin, who probably had received news more reliable from Archbishop Amo, and this time partly confirmed the allegations. Alcuin in a series of letters, however, did present to the king that in his view no one could submit the see of Rome to judgement and added that he considered harmful for the good of the Church, the deposition of a pope meant to discredit the office, while Charlemagne himself needed a credited pope. The monarch acted with great prudence in the secret negotitiations with the pontiff in Paderborn in the fall of 799. Pope Leo III returned to Rome accompanied by a large following of Frankish bishops and peers of France; the reception in Rome was triumphant. The Frankish diplomacy had worked well among the clergy, nobles and people of the city, putting the accusers in the minority. At the time, there was no judicial process; bishops gathered the acts of the investigation and sent it to Charlemagne along with the ringleaders of the rebellion, translated into the Frankish territory to avoid trouble. No sentence was emitted; Charlemagne would decide when to go to Rome. Once Charlemagne solved the problems with the Saxons, he went to Rome in November 800, with his son Charles and a large following of bishops and soldiers, carrying even the accusers of Pope Leo III; the other son, Pepin, continued to the south to wrest the duchy of Benevento from Grimolado. On 23 November, 800, the pope went to meet the monarch in Nomento, twelve miles from Rome, with the clergy, the people and the militia, and then they went back to the city; the next day Charlemagne entered solemnly in St. Peter's basilica, among a cheering crowd of prelates. On December 1, the clergy, nobles and Frankish and Roman citizens were summoned by the king to parliament and the assembly met in St. Peter. Charlemagne explained that he had gone to Rome to restore order in the Church; and that he would listen to citizens' protests against the pope and then would decide whether or not Pope Leo III was innocent. His assessment was unquestionable and the pope would appear in the king's court as any subject would do. In the discussions that followed for three weeks, while on the one hand the accusers failed to bring evidence to the king and on the other hand, the very case of the pope did not appear very clear; the position advocated by the Frankish bishops, on which evidently, even from afar, influenced AIcuino, who had preferred to remain in his convent of Tours, was consistent: they believed that the Apostolic See could not be judged because being it being in charge of all the Churches of God, we ourselves are judged by the pope, its representative on earth, they sais to clarify. The pope realized that the only way out was to submit himself to a oath of purgation, as it had done Pope Pelagius I before Narses; this move had been planned in Paderborn. Therefore, on December 23, 800, the pope, attended by Charlemagne and his peers, before a large crowd of clergy and people, with the Gospel in his hand, called to witness God "before whose court all must appear" that he had not performed nor had made anyone to do any the crimes of which he was accused. The oath of purification was considered sufficient to prove the innocence of Pope Leo III; his opponents were accordingly found guilty of the crime of treason and sentenced to death. The sentence was not performed through the intercession of the pope, who evidently feared that the death penalty would have made even more hostile certain segments of the population; the conspirators ended up in exile in Frankish territory. Two days later, on Christmas Day, Charlemagne, who was present at the celebration in St. Peter's basilica, had just finished praying and was about to stand up when Pope Leo III placed on his head a splendid golden crown and the people present gave the triple acclamation that usually accompanied the imperial coronation: "To Charles, most pious Augustus, crowned by God, the great emperor bringer of peace, life and victory." Certainly the crowd was not "inspired by God and the blessed Peter, the guardian of the gate of heaven," as the Liber Pontificalis indicates; the pope by prostrating himself before Charles to give him by that act the homage given to the ancient emperors, had taken a sudden historic decision; and even less can be suppose that Charlemagne was not prepared to receive the crown. The imperial coronation gave the pope unparalleled prestige and a special place, full of significance, even if it meant increasing even further the interference of Charlemagne in ecclesiastical affairs.