The first church or oratory on this site was built in wood in the 6th-century, and a replacement building was erected in around 970. Nothing remains today of those. The 10th century saw many attacks on Saint-Brieuc by the Normans, so much so that the relics of Saint-Brieuc which had been kept here were moved to the Saint-Serge Abbey in Angers for protection.

The building we see today was begun about 1180, and building was well advanced when in 1212, Bishop Pierre was buried at the base of the Tour Brieuc. Pierre had brought the Saint-Brieuc relics back to the town from Angers in October 1210. After a fire in 1353, it was almost entirely rebuilt reusing many old materials, but was barely finished when severely damaged by a siege by Olivier de Clisson in 1394. It was then again restored and, in large part, rebuilt. The north wing of the transept, the gable of the south wing, the chapel of the fonts, the mid-tower and the upper parts of the western gable and the western gable, date from the beginning of the 15th century. At the beginning of the 18th century, the building was disfigured by the total reconstruction of the nave. Many of its treasures were stolen during the French Revolution. Finally, in the 19th and 20th centuries, important restoration works were carried out, including the rehabilitation of the vaults of the transept, the choir, as well as the replacement of most fenestrations.

It has the look of a fortress, and did, in fact, fulfill the role of a church-fortress ("cathédrale-forteresse") on many occasions when the town was laid siege to in earlier times. The central porch is flanked by two sturdy towers: the 14th-century Tour Brieuc, 92 feet high, and the 15th-century Tour Marie, 108 feet in height. These towers have loop-holes and machicolations which would have allowed the use of a variety of defensive weapons and are supported by stout buttresses. The two arms of the transept jut far out and are protected by towers with pepper-pot roofs.