Monticelli, appears in an act of the Archives of Cassino, at the time occupied by Crescenzo, son of the deceased Giovanni, Consul and Duke of Fondi. The Act specifies that Crescenzo, inhabitant of the Fortress of Monticelli, sells to Docibile Gattula a piece of land to build a house in Castro Asprano. This is how the Castle of Monticelli and later the University of Monticelli were born. The history of this town is therefore bound with that of Fondi with which it often gets confused. In 1145, the fief of Monticelli passed under the rule of the Dell'Aquila Family and when the male descendants estinguished the fief was entrusted to Loffredo Gaetani's wife, Giovanna Dell'Aquila. The most well - known rappresentatives of the family were Onorato I and Onorato II. The dominion later turned over to the Colonna, then to the Carafa, to the Mansfeld and finally to the Di Sangro Family. When Monticelli became the most important way of communication of the Reign of Naples, during the winter of 1788, it underwent the invasion of the French Troops that set on fire and devastated the archives of the town hall and of the Parish.

Another important period is when the town was victim of the "Brigandage" (1860 - 1870), when many citizens paid with their lives because they refused to collaborate. The last heir that conserved the title of "Prince of Monticelli" was Giuseppe Di Sangro, against whom the town hall brought legal action for the reinstatement of community lands.

After Garibaldi's Expedition, in 1860, Monticelli became integrant part of the Unitary State. In 1862, the Prefect, invited the Mayor to modify the towns name because it was easily confused with that of Monticelli of Rocca Guglielmina (Esperia); therefore, denominated Mount Saint Vito. It was ordered to change also this name because mistaken with another town in the province of Ancona. On Dicember 20th, 1862 the Town Coucil decided for Monte San Biagio in honor of its Patron Saint Biagio. The decision was approved with Royal Decree on January 18th, 1863.