"A parish in the Union of St. Asaph, hundred of Isaled, county of Denbigh, 2 miles (NW) from Denbigh, containing 2703 inhabitants. This parish, which is fifteen miles in length, and in some parts more than seven in breadth, though averaging about four, extends from St. Asaph, on the north-east, to the source of the river Alwen, near Llanrwst, on the south-west. ..... The scenery is finely diversified, and from some of the higher grounds are obtained extensive and delightful views of the surrounding country, which is in many places characterised by features of great interest and beauty." [A Topographical Dictionary of Wales, 1833 & 1849, Samuel Lewis]
Henllan is one of the "ancient parishes" of Denbighshire, originally comprising the townships of Banister Isaf, Banister Uchaf, Brynysgub, Erifiat, Gwenynog Llan, Lleweni Isaf, Lleweni Uchaf, Rhanfawr, Rhanhir (or Twysog), Taldrach, Tre'rparc, and Uwchcaerau (or Uwchcaeron).
By an Order in Council, on 21 July 1855, more than half of the parish went to the new parishes of Trefnant and Bylchau. At a later date, a further part of the parish was lost to the parish of Denbigh.