Llawhaden and its larger hinterland was the property of the Bishops of St David's in the later years of Deheubarth's existence. The Norman Conquest of Deheubarth left the rights of the Bishops largely unchallenged, creating an effectively independent state under the Bishop's control. In 1115, after Bishop Bernard accepted the King's suzerainty over the land, King Henry I issued a charter formally acknowledging the lands of St. David's as a Marcher Lordship - Dewisland - of which the area around Llawhaden formed a detached part. As Marcher Lords, the Bishops of St. Davids had judicial powers over all offences, could levy tax, issue charters, raise armies, and start wars. Bernard built a castle at Llawhaden to protect the Llawhaden portion of his land, and the village grew up around it. As the chief seat of the detached part of Dewisland, he granted the settlement at Llawhaden the status of a burgh. As the lands north of the Preseli Hills were less fertile, and hence less hospitable, than those to the south, successive Bishops came to base their administration at Llawhaden; by the 13th century the exchequer, chancery and court of Dewisland had moved here. Though part of Dewisland, and ruled by the same person - the Bishop of St. Davids - the detached nature of Llawhaden and its hinterland lead to it being sometimes called the Lordship of Llawhaden, as if it were a distinct Marcher Lordship. King Henry VIII's 1535 Laws in Wales Acts abolished the powers of Marcher Lordships; Dewisland was merged by the Act with the adjacent Lordship of Pembroke, to form Pembrokeshire. The detached Llawhaden portion of Dewisland became a mere barony, and Llawhaden burgh ceased to have any administrative importance. George Owen, in 1603, described Llawhaden as one of nine Pembrokeshire "boroughs in decay".. The settlement at Llawhaden expanded slightly across the Eastern Cleddau river, and provided the nearest church for lands to the east - part of Gwarthaf ; hence the parish straddled both sides of the river. Though the Llawhaden part of Dewisland was English speaking, like Dungleddy to its west, Gwarthaf was Welsh speaking; the bilingualism was noted by George Owen's 1603 description, who regarded Llawhaden as one of six parishes which were "bilingual". The Eastern Cleddau formed the linguistic boundary, which has been fairly stable since Norman times, although, perhaps in the early Industrial Revolution when people moved to seek work, Welsh-speakers started emerging in the area around Gelli woollen mill. Llawhaden Bridge, to the east of the village, spans the Eastern Cleddau and is a Grade II* listed structure built in the mid-18th century. There are a number of other listed structures in the parish.
