The Cistercian Abbey of Cwmhir, a daughter house of Blessed Virgin Mary of Whitland, was constructed in 1143 by three sons of Madog, the then Prince of southern Powys. However the first community failed because of the intervention of Hugh de Mortimer, Earl of Hereford. The advance of this Norman Earl in the harsh centre uplands of Wales caused the then resident monks at the time to withdraw to their mother house at Whitland.

In 1176 the Lord Rhys of Deheubarth reestablished the ' Abbey of the Long Valley ' by the Clywedog river in this harsh upland wilderness on land given by Cadwallon ap Madog. It was and still is, a bleak foreboding place; where in winter snow dances in harsh northeast winds that have swept down from across the Berwyn Mountains. A typical Cistercian Abbey, remote and lonely.

When Lord Rhys died in 1197 the Abbey once again saw a change in fortunes, for the Mortimers once again advanced west. This time however, they presented the Abbey with a Norman Charter. Those monks who felt that they could not live under Norman rule set of north west to cross the Cambrian mountain range and establish another Cistercian Abbey in the wilderness north of the Pass of Corris at Cymer near Dolgellau. For those monks who remained however it was not all doom and gloom, as Mortimer replaced the old wooden structure with one of stone.

In 1228 when Llywelyn ap Iorworth ' Llywelyn the Great ' regained Welsh independence he began to turn the Abbey into a great national Cathedral. Llywelyn was friendly with the Corbett family, owners of the Grinshill stone quarries near Shrewsbury and the Mortimers of Hereford, so he imported Grinshill stone for the Abbey and some of the other great buildings. However, in 1231 he was thwarted in the completion of his plans. During the English King Henry III's skirmishes with the Welsh, a monk from the Abbey falsely directed a small English army across a bog near the Abbey, they not only became trapped but Llywelyn's forces lay in wait for them. When he was told of the treachery Henry retaliated by burning down a grange of the abbey and fining the abbot. However, that same year many privileges were granted to Cwmhir, but with the condition that the monks "did not abuse their liberty by assisting the king's enemies in Wales"

After the death of Llywelyn ap Gruffudd the last Prince of Wales at Builth in December 1282, his decapitated body was smuggled to the Abbey. When a few of the surviving commander's of Llywelyn's army succeeded in reaching the Abbey, they took his body into the grounds during the dead of night and buried him with a solemn oath that they would never disclose his resting place, no one ever did that I know of.