St Mary's Church is medieval in origin, with its first documentary reference dating to the twelfth century. It is situated within an irregularly shaped churchyard, whose east and north-east boundaries are curvilinear. The remaining boundaries are linear, and reflect later extension and encroachment. There is a raised area with a low bank to the north of the church. There are also traces of a low curvilinear bank centred on the church, within the north-west side of the churchyard. This suggests that the present north-west boundary may have encroached on the original churchyard area. The churchyard was extended to the north in 1919. The foundations for a north chapel adjoining the chancel were noted in the churchyard in 1851. The lychgate is located within the south-west boundary and dates to the eighteenth century. A holy well, Ffynnon Fair (NPRN 32381), is situated 260m to the east of the church. In 1894 a more ancient well, in ruinous condition, was reported to be located downslope, nearer to the church. The distribution pattern of groups of churches along the seaboard of Ardudwy has been noted, the churches in each group being in close proximity to one another. St Mary's, along with St Tanwg's, Llandanwg (NPRN 43901) and Llanbedr (NPRN 43897), are one such group, being located within about a mile and a half of one another.

The church is a grade 2 listed building consisting of a nave (formerly a continuous nave and chancel), a later undivided chancel and a north vestry. The eastern part is thought to be late fourteenth or early fifteenth century, and the walls are constructed of roughly coursed, even shaped rubble stone. The original east window has been retained. The western part is later, and is constructed of mixed stone. The roof reflects the two phases of construction, with the six eastern trusses being much heavier than the six western ones. A north chapel was added around 1500. The octagonal font is sixteenth century and the rood screen dates to the seventeenth century. The doorway into the north chapel is recorded as being known as `Drws Gwyr Nanmor? (Nanmor being a township in the parish of Beddgelert). The north chapel may have been demolished around 1765. A rood stoup constructed from hard local stone is located in the north wall. This may indicate the position of a former doorway. The church was whitewashed in 1742. The building was restored around 1857, at which time it was re-floored. Buttresses were added. A west doorway was substituted for one in a side wall. The west bellcote and most of the openings date to this time. The vestry is nineteenth century and is situated on the site of the old north chapel. A reset window in its north wall may be from the former chapel. The rood screen is noted as having been built by a local carpenter around 1877, but incorporates elements of the former rood screen. The bell was replaced in 1749. The church contains mural tablets to the memory of the Owens of Crafnant, descendants of Cynfyn, Prince of Powys.

Sources include:
Beverley Smith, J, Beverley Smith, Ll, 2001, History of Merioneth II, 353-354
Gwynedd Archaeological Trust, 2000, Historic churches of Gwynedd: gazetteer, 391
Davidson, A, 2008, `Two new early medieval stones from Llandanwg?, Archaeology in Wales 48, 73-75
Richards, R and Lloyd, R G, 1894-5, Transactions of the cymmrodorion Society, 124

RCAHMW, May 2012