Records of the history of the diocese before Norman times are very fragmentary, however, consisting of a few chance references in old chronicles, such as 'Annales Cambriae' and 'Brut y Tywysogion' (Rolls Series).

Originally corresponding with the boundaries of Dyfed (Demetia), St Davids eventually comprised all the country south of the River Dyfi and west of the English border, with the exception of the greater part of Glamorganshire, in all some 3,500 square miles (9,100 km2).

The early ecclesiastical organisation of the Welsh church is unclear but scanty references reveal that some form of archbishopric definitely existed, with multiple bishops under the jurisdiction of a senior see.

It is unclear when St Davids came definitely under the metropolitan jurisdiction of the Archbishop of Canterbury, but about 1115 King Henry I intruded a Norman into the see, Bernard, Bishop of St Davids, who prior to his ordination was confirmed by Canterbury, much to the disgust of the Brut y Tywysogyon which noted that Henry I 'made him bishop in Menevia in contempt of the clerics of the Britons’.

The failure of Gerald's campaign saw the claim lapse but it was revived by Owain Glyndŵr's plan for an independent Welsh Church.

In the troubled times of the Reformation the former bishop of St Davids, William Barlow (1536–1548), was a consecrator of Archbishop Matthew Parker in 1559.