There is a tradition that the present site was not the original location of St Tydecho's Church. It is thought to have been founded at Llandybo or Caereinon Fechan. The church is situated within a curvilinear churchyard on a rock outcrop, bounded by a farm track to the south, a road to the west, an old heating house to the north-west, and a cottage and garden to the north-east. It has two entrances, with its main gate in the south-west boundary, and the other in the north-east. A yew tree is located in the south of the churchyard, and another felled yew tree is noted to the south-east of the chancel, adjacent to the head of a three light window dated 1613. A chantry alter is known to have existed in the church at the close of the reign of Henry VIII. St Tedecho's, Mallwyd shares its dedication with St Tydecho's, Llanymawddwy (NPRN 43906). Llanmawddwy is the principal church, with Mawddwch being its subsidiary chapel. Together the two parishes formed the commote of Mawddwy, which did not become part of Merioneth until 1536.

The church is a Grade II listed building, and is orientated almost north to south. The style of the building is more closely related to neighbouring Montgomeryshire than that of Merioneth and consists of a continuous nave and chancel, south porch, west vestry and west wooden bellcote. A change in wall thickness signifies that the nave is earlier than the chancel. The nave is medieval in date, as is its roof, whose trusses are constructed of massive timbers with arch moulded tie-beams. A blocked slit window in the south wall is thought to be medieval, as is the present south door. A blocked south door is thought to be medieval, and the other blocked south door to be sixteenth century.

The church was modified in the seventeenth century by Dr John Davies of Mallwyd (1567-1644). Davies was a noted scholar and lexicographer who aided William Morgan in his translation of biblical texts. The modifications are thought to be part of a Laudian-inspired rebuild, and are considered of considerable architectural and liturgical interest. The porch is dated 1641. The chancel is seventeenth century, and has a wooden ceiling supported on curved principals. The feet of the brackets belonging to the second principal are carved. The southern one shows a lion (presently painted red), while the northern one shows a unicorn (painted white) with a fleur-de-lys (painted gold) below it. The tower contains three bells dated 1642, 1685 and 1738. At the east end of the south wall is another blocked door, and two seventeenth century windows, which were lengthened into dormers in the nineteenth century. A window in the north wall is seventeenth century in date, but has also been extended into a dormer. The small octagonal font is of black marble, and dates to 1734. The nave's late medieval roof trusses were mutilated by the removal of the tie beam and struts below the collar beam in the eighteenth cetury. Two north windows are eighteenth century and one is modern. Above the porch door are two bones, said to belong to a prehistoric mammal, which were dug up in Cae Llan sometime before 1914. A new board on the south side of the tower, bares the inscription 'SOLIDEO SANCRUM ANNO CHRISTIMDCI', and is a reconstruction of an older one.

Sources include:
Beverley Smith, J, Beverley Smith, 2001, History of Merioneth II, 365-366
Davies, C (ed), 2004, 'Dr John Davies of Mallwyd: Welsh Renaissance Scholar', chapter 10
Gwynedd Archaeological Trust, 2000, Historic churches of Gwynedd: gazetteer, 391

Source: Richard Suggett, Painted Temples: Wallpaintings and Rood-screens in Welsh Churches, 1200–1800, (RCAHMW 2021), pp. 236–8, 286.

RCAHMW 2022