"A parish in the Union of Llanfyllin, composed of the lower division, in the Cynlleth and Mochnant division of the hundred of Chirk, county of Denbigh; and the upper portion, in the upper division of the hundred of Llanfyllin, county of Montgomery; 12 miles (W by S) from Oswestry, and containing 2344 inhabitants. ..... This parish is intersected by the river Moch, which, at the distance of four miles from the village, forms the much admired waterfall called Pistyll Rhaiadr. ..... The river precipitates itself with great impetuosity down a steep descent of more than 150 feet, and, being interrupted in its fall by a projecting mass of rock, through which it has formed a channel, forms a second fall beneath a lofty arch to the base of the mountain." [A Topographical Dictionary of Wales, 1833 & 1849, Samuel Lewis]
According to Archdeacon Thomas, the parish of Llanrhaeadr ym Mochnant at one time included the parishes of Llanarmon Mynydd Mawr, Llangadwaladr, Llangedwyn; and part of Llanwddyn (in Montgomeryshire).
The townships of the parish of Llanrhaeadr ym Mochnant which were in the pre-1974 county of Montgomery were: Abermarchnant, Brithdir, Castellmoch, Cefn Coch, Glanhafon Fach, Glanhafon Fawr and Nantfyllon.
The townships which were in the pre-1974 county of Denbigh were: Banhadla Ucha, Banhadla Isa, Gartheryr, Henfachau, Trebrys Fawr, Trebrys Fach, Trefeiliw, Tre'r-Llan and Tre-Wern.
The parish church was on the Denbighshire side of the border; therefore the parish registers have been deposited in the Denbighshire Record Office, Ruthin.
The village is perhaps best known as the place where William Morgan, who was vicar from 1578 to 1588, completed his translation of the Bible into Welsh.