St. Leo IV received his religious education in Rome at the monastery of St. Martin. Because of his piety, Gregory IV made him a subdeacon, and later, Sergius II made him a Cardinal-Priest of the church of the Quatuor Coronati.

In 847, Leo was consecrated as pope, succeeding Sergius II. The year earlier, the Saracens had attacked Rome, and tensions were high in the city. Leo immediately began rebuilding the walls around the city to prevent future attacks, and he was the first to enclose the Vatican hill within a wall. Leo also built the church of S. Maria Nova.

In 853, Leo held a synod in Rome, during which he passed decrees regarding ecclesiastical discipline and learning, and condemned the actions of Anastasius, Cardinal of Marcellus.

St. Leo IV died in 855 and was buried in St. Peter’s on July 17.