The area contained the former Samnite settlement of Cereate Marianae, now called Casamari, but the current village was formed in the Middle Ages when the populations took refuge in castles on high ground to escape the Barbarians. The first news of the castle dates back to the early decades of the eleventh century, when the castle was under the control of Bishop of Veroli and it was called Castelforte.
The current name comes from that of a church, a small Benedictine monastery founded in the sixth century and dedicated to San Giovanni, built over an earlier temple dedicated to Janus (Giano). To the name of Monte San Giovanni was added, in about one thousand, the nickname Campano to indicate that the country was in the papal province of Campagna.
In 1243 the castle was in the hands of Landolfo, the father of St. Thomas Aquinas, who was locked up for about two years. The Aquinas lost the estate in 1422 for a few years, when Martin V confiscated it and gave it to his relatives. Around 1471 the estate became part of the Kingdom of Naples. In 1495 Charles VIII of France occupied these territories of the Pontifical State and destroyed the castle.
At the beginning of the sixteenth century Pope Alexander VI confiscated the estate and gave it to his nephew Rodrigo Borgia. In 1568 the estate, created a duchy by Pius V, was returned to Aquinas d’Avalos and Antonella d’Aquino Avalos who erected a Capuchin monastery over the former church of the Benedictine monastery.
In 1595 Pope Clement VIII recovered the fiefdom of Monte San Giovanni and others from Aquinas d’Avalos and brought it together with others to bring order to the southern borders of the state, with the achievement of geographical boundaries along the Liri River Hills and Strangolagalli. The pope granted a specific general amnesty that even caused the destruction of the trial files for cases already in court and those judgments from cases that had already been completed. Since that time Monte San Giovanni was included in the Papal defence system.
In 1798 Monte San Giovanni was occupied by French troops but the population sided with the pope-king. The 1915 earthquake severely damaged the old castle and the remaining houses in the town. It is counted among the most beautiful towns in Italy.