In 1128 Fergus of Galloway appointed Gilla-Aldan as bishop of Whithorn. This re-established the bishopric here, which had lapsed under Viking rule. Changes began to take place.

It seems that Fergus initially placed a community of Augustinians here. Augustinian monks followed The Rule of St Augustine. They believed in quiet contemplation, but also in mixing with the community and ministering to them. The Augustinians did not last long at Whithorn. They were replaced by Premonstratensians in around 1175, possibly from nearby Soulseat Abbey. This may have been done as an attempt to introduce a more regulated form of monastic life.

The Premonstratensian order takes its name from the abbey of Prémontré, in north-eastern France, founded by Norbert of Xanten in 1121.

They are also sometimes known as Norbertines and, in Britain, as ‘the white canons’, because of their white robes. Norbert followed the Rule of St Augustine, but was also a close associate of St Bernard, an influential advocate of the Cistercian way of life, and so the canons obeyed additional statutes, as well as the Rule, which made their life one of great austerity. The order was probably first introduced to Scotland at Dryburgh Abbey in 1150. And so Whithorn became a cathedral priory, where the bishop shared his church with a monastic community.

Most surviving structures at Whithorn were built during the Premonstratensian period. They built cloisters and a great, cross-shaped cathedral on the site of the earlier church to accommodate their rounds of prayers.