St Mary's church lies on a low hill accessed by a short lane off a sharp bend in the minor road from Trecwn to Letterston. The church is situated within a curvilinear churchyard and the site may have early medieval origins. There are a number of earthworks within the churchyard and it has been suggested that a large recumbent limestone slab to the north-west of the church may have been reused as a leacht (i.e. a `blessing? or `alter stone?). The church was first recorded in documents of 1125, when it was granted by Robert FitzHumphrey to Slebech Commandery. The church was known as `chapel of St Mary Magdalen in Llanvair? in 1330, when John Lettard of Letterstonreleased all his rights as lord of the manor to the Knights Hospitaller. At the dissolution the church, along with all the appurtances of Slebech Commandery, fell to the Crown. By 1594 the church was a chapelry annexed to the rectory of Letterston. By 1833 the church wa a parish church, but its living was still annexed to Letterston. Llanstinan parish church was formerly a chapelry belonging to Llanfair Nant-y-Gof.

The form of the earlier church is not known, although the present church is on the same site and in the same location. The present neo-gothic church was rebuilt in 1855 to the designs of R.J. Withers. It is constructed of limestone rubble and consists of 4-bayed nave, 2-bayed chancel, south transept, north porch, lean-to vestry and gabled bellcote. All dressings are in oolite. The nave west wall carries the bellcote, in limestone ashlar with a two-centred opening. The lancet windows have plain, two-centred surrounds. Fittings include a square font (dated 1695) and poppyhead pews, and some monuments from the earlier church. Patterned glass includes works by Powell (1855) and Celtic Studios (1975).

Sources include:
Dyfed Archaeological Trust, 2000, Historic Churches Project, Pembrokeshire gazetteer
Dyfed Archaeological Trust, 2003, Early Medieval Ecclesiastical Sites Project, Pembrokeshire gazetteer
T Driver, 2007, 'Pembrokeshire: Historical Landscapes from the Air' RCAHMW, page 55, Fig. 83.
T.Lloyd, J.Orbach & R.Scourfield, Buildings of Wales: Pembrokeshire (2004)

N Vousden 3 January 2018