It currently covers most of the former Welsh county of Glamorgan, but once stretched from the River Towy to the middle of the Wye Valley.

Lucius was believed to be a 2nd century king who first beseeched the Pope (Eleutherius) to convert him to Christianity.

The Welsh Triads relate this tradition to Llandaff, stating that Lucius "made the first Church at Llandaf, which was the first in the Isle of Britain."

another triad lists "the three archbishoprics of the Isle of Britain" and states that "the first was Llandaf, of the gift of Lleirwg (Lucius), the son of Coel, the son of Cyllin, who first gave lands and civil privileges to such as first embraced the faith in Christ."

Although the Lucius legend is now considered to be pseudohistory, it was recounted by Nennius, Bede and Geoffrey of Monmouth, and seems to have been widely accepted in the medieval period.

[2] Four names are associated with the task of executing the Pope's wishes, these include the early Welsh saints Fagan, Deruvian and Elvan.

The early history of diocese is, however, highly controversial, because the chief authority, the diocesan charters in the Book of Llandaff, though dating from the late 6th century, were considerably doctored to raise Llandaff's profile when they were copied in the early 12th century.

Gilbert Hunter Doble and others have clearly demonstrated that there is no evidence that Saints Dubricius and Teilo had anything to do with Llandaff.

Dubricius was only active in Ergyng and Gwent, while Teilo's associations with Llandaff have been transferred from his great abbey at Llandeilo Fawr.

The early episcopal authority in the area was, indeed, in Ergyng and Gwent, originally under Dubricius and then his disciples.

Standing difficulties were the admixture of race and language due to the English settlements and the ignorance and incontinence of the Welsh clergy, who had ceased to observe celibacy and gave scandal to the Normans and English alike.

The Book of Llandaff, now at the National Library of Wales, Aberystwyth, reflects Urban's territorial ambitions for his diocese.

Rome had already decided to suppress the Catholic see in 1530; Thereafter, there continued a line of Anglican bishops up to the present day.

Some of these showed aptitude for the conditions of the post, e. g. Blethyn and Morgan (the translator of the Bible), also those appointed under Charles II.

The administration of the diocese suffered from its poor endowment and limited patronage, leading at the end of the 18th century to non-resident bishops (e.g. Watson) and the holding with other ecclesiastical benefices (such as the Deanery of St. Paul's).

Bishop Ollivant notably took up the challenge of providing churches for the newly industrialised valleys.

The population explosion created pressure for the division of the diocese, which was put into effect with Disestablishment.

The diocese remained part of the Province of Canterbury until the creation of the Church in Wales on 31 March 1920.

[17] This was formed by the union of the parishes of Capel Llanillterne, Michaelston-Super-Ely, Pentyrch, Radyr, St Fagans and Tongwynlais.

[23] 1original dedication to St James This was formed by the union of the parishes of Gilfach Goch, Tonyrefail and the Llantrisant Rectorial Benefice.

[38] 1originally dedicated to St Mary[42] This was formed by the union of the parishes of Cardiff Roath Park, Lisvane and Llanishen.

[47] 1originally known as Christ Church Llanishen This was formed from the benefice of Pontypridd,[48] previously including the parish of Pwllgwaun and Llanddewi Rhondda.

[81] 1original dedication to St Samson This was formed by the union of the parishes of Bargoed, Bedlinog, Brithdir, Deri, Fochriw, Gelligaer, Llanbradach, Llanfabon, Pontlottyn, Treharris, Trelewis and Ystrad Mynach.

[111] This was formed by the union of the parishes of Llansawel Briton Ferry, Skewen and the Neath Rectorial Benefice.

[117] 1original dedication to St Thomas a Becket 2known as the Tonmawr Mission until 2003 This was formed from the Rectorial Benefice of Barry,[124] including the former parishes of Merthyr Dyfan and Cadoxton-juxta-Barry.

[126] 1original dedication to St Teilo This was formed in 2022 by the union of the parishes of Cadoxton-Juxta-Neath, Cilybebyll, Dulais Valley, Dyffryn, Tonna and Vale of Neath.

[136] This was formed by the union of the parishes of Colwinston, Llandow, Llysworney and the Cowbridge Rectorial Benefice.

[146] This was formed by the union of the parishes of Aberaman, Abercynon, Cwmaman, Cwmbach, Matthewstown, Miskin, Mountain Ash, Penrhiwceiber and Ynysboeth.

[193] 1this church was located in Kenfig and was moved (literally) to Pyle in the 15th century due to encroaching sands This was formed by the union of the parishes of Aberkenfig, Bettws, Llandyfodwg & Cwm Ogwr, Llangeinor & the Garw Valley, Llanilid, Llansantffraid Aberkenfig and Pencoed.

[211] 1occasional services still held This was formed by the union of the parishes of Coity, Coychurch, Laleston, Merthyr Mawr, Newcastle, Nolton and Penyfai.