It has been listed since 1862 as a monument historique by the French Ministry of Culture.

The cathedral is said to have been built atop the foundations of a chapel constructed in the 3rd century by Saint Saturnin, sent to Christianize the Gauls and martyred in Toulouse.

It is said to have been reconstructed by Saint Exuperius, Bishop of Toulouse, one hundred and fifty years later. This first documented cathedral is recorded at the beginning of the 5th century, but nothing remains of the original building.

A Romanesque cathedral was constructed on the same site beginning in about 1078. The Romanesque structure was smaller than the present church; it was probably about twenty meters wide and 85 meters long.

It probably had a massive west front with two towers, a nave with three vessels, and a chevet in three parts extending outwards. The lower portions had oculi at the west. It included works of classical sculpture, such as a votive altar, probably from an earlier church. Examples are now displayed in the Musée Saint-Raymond in Toulouse. This early building was probably begun by bishop Isarn (1071 to 1105), and was continued by his successor Amiel (1105 to 1139).