[9] The first known bishop is St. Eustasius, whose name coupled with Aosta is signed to a letter sent to Pope Leo I by the second Synod of Milan in 451.

[10] In the cathedral treasury is a consular diptych of Anicius Petronius Probus, Roman consul in 406, which shows the Emperor Honorius.

[11] Bernard of Menthon (1008), Archdeacon of Aosta, founded the hospice on the Alps named after him, as a relief to pilgrims.

The two titles were separated at or following the death of Bishop Anselm of Aosta, in 1026, owing to Conrad II's desire to strengthen his position near the important Little St Bernard Pass and distrust of Burchard, Anselm's successor and a relative of various nobles opposed to Conrad's claims in Burgundy.

His namesake, Anselm, Archbishop of Canterbury (1033–1109), was also a native of Aosta and probably related to its dynasty of bishops; however, rather than remain in local service, he travelled to Bec Abbey in Normandy and ultimately became primate over Norman England instead.

[14] In 1134 Bishop Herbert granted the Canons the free administration of their goods, releasing them from episcopal control.

In the Spring of 1536, John Calvin, the famous Protestant reformer, visited Aosta as he was returning to France from Ferrara.

Bishop Paolo Giuseppe Solaro di Villanova (French: Paul-Joseph Solar de Villeneuve) resigned, so as not to impede the operation of the Bull.

[19] The diocese of Aosta was re-established in 1817 by Pope Pius VII in his bull, Beati Petri (17 July 1817).

A twenty-first Canon was added on 14 September 1721, through the generosity of Father Jean-Baptiste du Chatelard, Prior Commendatory of the cathedral.

This system lasted down until the administration of Bishop Filiberto Alberto Bailly (1659–1691), who traded his right to nominate in February, May, August, and November to the Pope, in exchange for the right to appoint to the cures in his diocese.