The name of Viterbo occurs for the first time in the 8th century, under Pope Zachary, when it was a village tributary to Toscanella, in Lombard Tuscany (Tuscia Langobardorum) on the Via Cassia.

Among the successors of Maurus was Virbonus,[3] to whom Pope Leo IV addressed a bull on 23 February 852, determining the boundaries of the diocese.

[4] In 876, Bishop Joannes was one of the legates of Pope John VIII at the council of Pontigny, and carried the imperial insignia to Charles the Bald.

Bishop Richard (1086–1093),[6] however, adhered to the party of Frederick Barbarossa's antipope Clement III, who, in 1193, united Toscanella with Centumcellae and the see of Blera.

[9] In the fourteenth century the clergy of Toscanella repeatedly refused to recognize the bishop elected by the chapter of Viterbo, so that Pope Clement V (1312) reserved to the Holy See the right of appointment.

[12] In 1353, Cardinal Albornoz, who was appointed Legatus a latere and Vicar in spiritualities and temporalities for all the lands in Italy subject to the dominion of the Church, came to effect the reconquest of the Papal States.

[17] In 1375 Francesco di Vico took possession of the city, which joined in the general revolt against papal rule, but quickly submitted.

When the Western Schism arose, Vico's tyranny recommenced; he took the side of Pope Clement VII and sustained a siege by Cardinal Orsini.

[18] After a century of trouble, peace was not re-established until 1503, when the government of Viterbo was subsequently assigned to a cardinal legate, rather than to the governor of the Patrimony.

One of its cardinal legates was Reginald Pole, around whom there grew up at Viterbo a coterie of friends, Vittoria Colonna among them (from 1541 to 1547), who aroused suspicions of heterodoxy.

[20] The Second Vatican Council (1962–1965), in order to ensure that all Catholics received proper spiritual attention, decreed the reorganization of the diocesan structure of Italy and the consolidation of small and struggling dioceses.

Based on the revisions, a set of Normae was issued on 15 November 1984, which was accompanied in the next year, on 3 June 1985, by enabling legislation.

On 2 May 1936, Pope Pius XI issued a bull, suppressing the territorial abbey as an autonomous prelature, and united it to the diocese of Viterbo and Tuscania.