In a legend of high importance in Welsh tradition, the origins of St Melangell''''''''s church are traced to the C7/C8, when St Melangell (Latinised as Monacella), was thought to have saved a hare hunted by Brochwel, a Prince of Powys. The legend is represented in the frieze from the C15 screen. The Celtic dedication of the church, the rounded form of the churchyard and the location all point to pre-mediaeval origins. The legend also tells that St Melangell founded a nunnery here, but it did not survive to the start of mediaeval records.
Graves have been revealed underlying the north side and the apse of the present building. The present structure starts as a stone-built church probably built by Rhirid Flaidd, d.1189, perhaps including the shrine. The ''''''''ecclesia de Pennant'''''''' is first recorded in 1254. By this time it probably consisted of nave and long chancel possibly as one cell plus an apse which covered a prominent grave. (These latter important features have been revealed by excavation.) Pilgrimage to the shrine of St Melangell, a fine Norman monument dated architecturally to 1160-70, was well established by the C15; the shrine may have been in the chancel or in the apse; it was probably broken up in the Reformation. The font is also late C12 Norman, much worn.
The church has been much altered. A tower and belfry were probably added in the C17. Reconstruction of most of the north wall (and probably the west and much of the south) is recorded by a stone of 1635(?) with the initials TV.WP. The porch is dated 1737. A stone dated 1738(?) dates the south wall of the chancel. The former cell-y-bedd (grave chamber) against the east wall, replacing an earlier apse, was constructed in 1751. It had no communication with the church, but was entered directly from the churchyard; it was used as a vestry until the 1830s and as a schoolroom until superseded by the village school in Llangynog shortly after 1870. Inside, the nave roof is of the C15, and the rood screen and loft and its carving of the legend are probably of the same date, though relocated (the rood-loft was removed in the 1720s.) A window was inserted in the wall west of the porch to light the west gallery in 1721, but neither the window nor the gallery now survive. Glynne, visiting in 1848, found a mean exterior but some points of interest, noting especially ex-situ Norman architectural features built into the south wall. Inside he noted the carvings representing the legend of St Melangell, also the surviving rood screen with four compartments each side.
The church was restored in 1876-7 by Benjamin Lay of Welshpool, including the complete rebuilding of the tower. New windows were inserted in the south wall (east and west of the porch) and in the north wall. Wall paintings revealed during this work were not preserved, but the wood carvings representing the legend of St Melangell were removed to the front of the west gallery. During further restorations in 1951 the cell-y-bedd or grave chamber was refloored and the foundations of the earlier apse discovered. The shrine was reconstructed from fragments remaining in the walls of the church and the lychgate.
Following threats of demolition the church underwent a major programme of research and restoration in 1989-92, at a cost of £130,000, including the removal of the cell-y-bedd and its replacement by a small apse representing the Norman apse. The church was reordered by R B Heaton. The screen was moved about 2½ m west and placed on a stone plinth, and the beam and traceried loft formerly on the west gallery were restored in front of it. The space above was boarded, providing a surface upon which to display a bronze Corpus Christi on the nave side, by the artist Katherine Fuller. On the other (chancel) side are the Creed, Commandments and Lord''''''''s Prayer taken from the plaster of the east wall; a C19 replica is in the church tower. These texts are now only now visible within the chancel when looking west. The Norman font was removed to the south door. The most important change was the rebuilding of St. Melangell''''''''s shrine in the centre of the chancel, reusing all available original stone carvings. The C18 altar rails were moved and altered to make access to the shrine easier. The programme included touristic objectives and the fitting of the tower base as a shop and its upper storey as a gallery for local history and similar displays.