The town of Montefiascone, or, more specifically, the Rocca di Montefiascone, had long been the official residence of the Rector of the Patrimony of Saint Peter, and, whenever a pope visited, of the pope as well. Pope Urban V had stayed at Montefiascone during his journey from Avignon to Rome, and was greatly impressed by the loyalty and affection of the inhabitants toward himself and his predecessors. The Pope held a consistory for the creation of new cardinals at Montefiascone on 22 September 1368. He named six Frenchmen, a Roman, and an Englishman. The diocese of Montefiascone was erected by Pope Urban V by the papal bull, Cum Illius of 31 August 1369. He appointed the church of S. Margarita to serve as its cathedral, and he installed in it a Chapter composed of two dignities and eight Canons with prebends. The Dean was to be elected by the Chapter and installed by the bishop, but the other offices were to be filled by appointment by the bishop. The territory for the diocese was taken from the diocese of Bagnoregio, and any properties or rights within that territory which belonged to the bishops of Bagnoregio, Castro, Orvieto, Viterbo or Tuscano were assigned to the bishop of Montefiascone. Pope Urban held another consistory for the creation of new cardinals at Montefiascone on 7 June 1370. Two cardinals were named, one a Florentine and the other from Rodez in France. He departed from Montefiascone for Avignon on 26 August 1370, where he died on 19 December 1370.
New diocese
Its first bishop was the French Augustinian Pierre d'Anguiscen, appointed in 1376. In 1378, when the Western Schism began, Bishop Pierre became a partisan of Clement VII, and he was therefore deposed by Urban VI. On 5 December 1435, the diocese of Montefiascone was united with the diocese of Corneto in the person of the bishop; that is the bishop of Montefiascone was also at the same time the bishop of Corneto, with each diocese retaining its own institutional integrity. The union continued until, in 1854, Corneto became a part of the diocese of Civitavecchia. In 1483, Bishop Domenico della Rovere laid the cornerstone for the new cathedral of S. Margarita in Montefiascone, and in his Last Will and Testament in 1501 he left money to continue the work, which had barely reached the level of the main floor of the church at the time of his death.
Synods
Cardinal Marcantonio Barbarigo held a diocesan synod in the cathedral of Montefiascone on 1–3 June 1692. Bishop Lodivio Zacchia held a diocesan synod in 1622. Cardinal Jean-Siffrein Maury held a diocesan synod. Bishop Sebastiano Pompilio Bonaventura presided over a diocesan synod on 16–18 June 1710. The erection of the diocesan seminary for Corneto and Montefiascone was the work of Cardinal Marcantonio Barbarigo.
End of the diocese
By the middle of 1986, papal policy in the selection of bishops had concentrated in the person of Bishop Luigi Boccadoro: the Diocese of Viterbo e Tuscania, the diocese of Acquapendente, the diocese of Montefiascone, and the Administratorship of the diocese of Bagnoregio ; he was also the Abbot Commendatory of Monte Cimino. On September 30, 1986, Pope John Paul II moved to consolidate these several small dioceses by suppressing them and uniting their territories into the diocese of Viterbo e Tuscania, whose name was changed to the Diocese of Viterbo. The diocese of Montefiascone ceased to exist.
