Saint Leo was born in Rome and was the son of Radoald. Leo received his early education in Rome in the monastery of St. Martin, near St. Peter’s Basilica. His pious behavior drew the attention of Gregory IV, who made him a subdeacon, and he was later created Cardinal-Priest of the church of the Quatuor Coronati by Sergius II.
He was unanimously elected to succeed Sergius II as Pope. Leo was consecrated on 10 April, 847 without the consent of the emperor due to an attack in Rome by Saracens during the time of election.
As soon as Leo, much against his will, became Pope, he began to take precautions against the repetitious acts of the Saracen raid of 846. He began a project to put the walls of the city into a thorough state of repair, entirely rebuilding fifteen of the great towers. He was the first to enclose Vatican hill with a wall. In order to do this, he received money from the emperor, and help from all the cities and agricultural colonies (domus cultae) of the Duchy of Rome.
In the year, 853, he held an important synod in Rome, in which various decrees were passed for the furtherance of ecclesiastical discipline and learning, and for the condemnation of the refractory Anastasius, Cardinal of St. Marcellus, and sometime librarian of the Roman Church. Leo was buried in St. Peter’s on July 17, 855. He is credited with being a worker of miracles both by his biographer and by the Patriarch Photius. His name is found in Roman Martyrology.