Leo was a Roman, educated at the monastery of St Martin near St Peter's. Gregory IV made him subdeacon and Sergius II made him cardinal-priest of the Quattro Coronati. After the Saracens attacked Rome in 846, he was elected and consecrated in April 847 without waiting for the emperor's consent, much against his will.

He repaired the city walls, rebuilt fifteen towers, and for the first time enclosed the Vatican hill with a wall; the new quarter was called the Leonine City. In 849 a Saracen fleet heading for Rome was destroyed off Ostia by the allied fleets of Rome, Naples, Amalfi and Gaeta and by a storm.

He restored St Peter's and many other churches damaged in the raid, and built S. Maria Nova. In 853 he anointed the boy Alfred, the future King of England. He died in 855.

The Saracen fleets were masters of th<e Mediterranean, and Sicily was in the hands of the unbelievers. They had established themselves in Calabria, and were marchin g northwards, flaming towns and fields reeking with blood marked their progress. The Moslem was next at the gates of Rome, and had pillaged the suburbs, and the churches outside the walls had been defiled by the Mussulman, foot.

A Roman and the son of Radoald, was unanimously elected to succeed Sergius II, and as the alarming attack of the Saracens [medieval Christian name for Arab and Muslim peoples] on Rome in 846 caused the people to fear for the safety of the city, he was consecrated (10 April, 847) without the consent of the emperor. Leo received his early education at Rome in the monastery of St. Martin, near St. Peter's. His pious behaviour attracted the notice of Gregory IV, who made him a subdeacon; and he was created Cardinal-Priest of the church of the Quatuor Coronati by Sergius II. As soon as Leo, much against his will, became pope, he began to take precautions against a repetition of the Saracen raid of 846. He 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 the Vatican hill by a wall. 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. The work took him four years to accomplish, and the newly fortified portion was called the Leonine City, after him. In 852 the fortifications were completed, and were blessed by the pope with great solemnity.

Whilst the work of refortifying the city was in progress, a great fleet of the Saracens sailed for Rome, seemingly from Sardinia, but it was completely destroyed off Ostia by the allied fleets of Rome, Naples, Amalfi, and Gaeta, and by a tempest (849). When the rebuilding of the walls of Rome was accomplished, Leo rebuilt Portus, and handed it over to a number of Corsican exiles, whom the ravages of the Saracens had driven from their homes. Other cities too in the Roman duchy were fortified, either by the pope himself or in consequence of his exhortations. Leo also endeavoured to make good the damage which the Saracen raid of 846 had done to the different churches. St. Peter's had suffered very severely, and though as a whole it never again reached its former magnificence, Leo managed to make it in parts at least more beautiful than it had been before. St. Martin's, where he had been educated, the Quatuor Coronati, of which he had been the priest, the Lateran [St. John Lateran, the pope’s cathedral in Rome] Palace, the Anglo-Saxon Borgo, Subiaco, and many other places both in Rome and out of it were renovated by the energetic Leo. It was by this pope that the church of S. Maria Nova was built, to replace S. Maria Antiqua, which the decaying Palace of the Caesars threatened to engulf, and of which the ruins have recently been brought to light. In 850 Leo associated with Lothair in the empire his son Louis, by imposing on him the imperial crown. Three years later "he hallowed the child Alfred to king [says an old English historian] by anointing; and receiving him for his own child by adoption, gave him confirmation, and sent him back [to England] with the blessing of St. Peter the Apostle."

The same 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. Equally rebellious conduct on the part of John, Archbishop of Ravenna, forced Leo to undertake a journey to that city to inspire John and his accomplices with respect for the law. It was while engaged in endeavouring to inspire another archbishop, Hincmar of Reims, with this same reverence, that Leo died. Another man who, till his death (851), defied the authority of the pope was Nomenoe, Duke of Brittany. Anxious to be independent of the imperial authority Nomenoe, in defiance both of Leo and Charles the Bald, not only deposed a number of bishops, but made new ones, and subjected them to a metropolitan [the archbishop who heads a group of dioceses] see (Dol) of his own creation. It was not till the thirteenth century that the Archbishop of Tours recovered his jurisdiction over the Breton bishops. For consecrating a bishop outside his own diocese, St. Methodius, Patriarch of Constantinople, had suspended Gregory Asbestas, Bishop of Syracuse. St. Ignatius, who succeeded St. Methodius, in consequence forbade Gregory to be present at his consecration. This led Gregory to break all bounds. St. Ignatius accordingly caused him to be deposed, and begged the pope to confirm the deposition. This, however, Leo would not do, because, as he said, Ignatius had assembled bishops and deposed others without his knowledge, whereas he ought not to have done so "in the absence of our legates or of letters from us". Despite the fact that Leo was then in opposition to the Patriarch of Constantinople, one of his dependents, Daniel, a magister militum, accused him to the Frankish Emperor Louis of wishing to overthrow the domination of the Franks [the Germanic people who ruled Gaul and gave France its name] by a Greek alliance. Leo had, however, no difficulty in convincing Louis that the charge was absolutely groundless. Daniel was condemned to death and only escaped it by the intercession of the emperor. Shortly after this Leo died, and was buried in St. Peter's (17 July, 855). He is credited with being a worker of miracles both by his biographer and by the Patriarch Photius. His name is found in the Roman Martyrology.