It is the northernmost of the four suffragan dioceses in the ecclesiastical province of the Metropolitan Archdiocese of Santiago de Compostela, which covers Galicia in the northwest of Spain.

It seems to have been founded by the Suevian king, Theodomir, converted to Catholicism by St Martin of Dumio, and to have included in its jurisdiction the churches of the Britones (a territory coinciding with that of Mondoñedo) and some of those of the Asturias.

After Mailoc no mention is found of the bishops of Britonia for a long time, doubtless because the great distance from Toledo made it impossible for them to assist at the councils.

It may also be noted that, in the repartition of the parishes, the church of San Pedro de Nova was assigned as the residence of the bishops of Britonia and Orense, when they should come to assist at the councils of Oviedo.

After the time of St. Martin it was transferred to Villamayor de Brea, from which it derived the name of Villabriensis, and afterwards to Ribadeo, but it was nevertheless known as Mindoniense, as a document of the year 1199 bears witness.