St Mary's Church, Llanfair-ar-y-bryn, is located within a Roman site, thought to the auxiliary fort of Alabum which was established in the 70s AD during the Flavian advance. The Roman site consists of a rectangular enclosure (measuring some 180m north-east to south-west by 140m), comprising two sections separated by a road (which branches north from the A483 and joins it again some 400m further north-east). The enclosure's well preserved western section, located to mainly to the north and west of the church, is defined by scarped banks. The eastern section extends some 70m south of the church, and is delineated by the A483 to the east.

From around 1126 the church was a cell of the Benedictine priory at Great Malvern, but was dissolved by Rhys Grug. St Mary's is located within the parish of Llandingat, but was a parish church to Llanfair ar y Bryn (around 1.5 km to the north) during the post conquest period, belonging to the Deanery of Stradtowy. In 1710 the church was referred to as a chapel to Llandingat.

The church ceased to be a parish church in 1883 when a new parish church, St Mary's Church (NPRN 418661), was opened at Cynghordy (some 5.5km to the north-east). St Mary's then became a chapel of ease to St Dingat's Church, Llandingat (NPRN 104065). In 1998 the church belonged to the Rural Deanery of Llangadog and Llandeilo.

The church is situated within a curvilinear churchyard, bounded by a road on its north-west side and entered via a lych gate in its south-west boundary. The church walls are noted to have fragments of Roman tile within them. The vestry houses the desk of William Williams, Pantycelyn, hymnodist and preacher (1717-1791), dated 1775. His tomb and memorial (NPRN 467) are located in the northern section of the churchyard. A post conquest inscribed stone and moulded head are set into the internal face of the nave north wall. George Borrow, renowned travel writer and author of 'Wild Wales', attended a service here in 1854.

The church is a Grade I listed building, constructed of local mixed rubble stone. It consists of eight bayed nave and chancel, south porch, four-storeyed west tower and vestry and organ chamber (north of nave). Dressings are of Roman tile, limestone and old Red Sandstone (medieval), yellow oolite (1880) and Old Red Sandstone (1880 and 1913).

The nave and chancel is thought to date to the twelfth century, and has small slit lights. It appears that a transeptal structure led from the north wall, but this had gone when the thirteenth century tomb recesses were inserted. The church was reportedly restored in 1290, at which time the east bay was raised and the former south chapel constructed. In 1484 Richard III donated 10 marks towards repair of the St Mary's and Llandingat Church. The late fifteenth century windows, tower and insertion of a rood screen are thought to date to this time. The original south porch is thought to have been constructed in the fourteenth century. The octagonal font bowl, of Old Red Sandstone, is fifteenth century (with modern limestone stem). The west tower is late fifteenth century in date. There were four bells in 1535.

There is a remnant of a pre-Reformation wall painting, consisting of chevron panel border with traces of a polychrome figure, which was noted in 1973. Another, consisting of palimpsest text, possibly English black letter Decalogue (Ten Commandments), overlying an earlier red-yellow border or window, is thought to be sixteenth to seventeenth century in date.

In 1710 the area between the south chapel and porch was noted to measure some 25ft in length and to be full of skulls and bones which were piled some 6ft high against the external church wall. Much of the exterior walls were ivied at this time.

In 1781 the chapel was referred to as Llwynhowell Chapel. By 1790 it was noted to be ruinous, and is thought to have been demolished soon after. In 1854 the walls were noted to be painted yellow and there was a pulpit on the north wall. The vestry is thought to be early-mid nineteenth century. Medieval window openings are thought to have been blocked, altered or re-fenestrated around this time.

The church was restored in 1880, at which time there was some re-fenestration, the organ chamber was added, and the porch rebuilt. The tower was restored in 1906, and the church in 1913, to the designs of W.D. Caroe. He criticised the previous restoration and noted that the late fifteenth century west window had been reset upside down and capped with modern work. Medieval openings were unblocked, including the sills of the original east window. Other windows were rebuilt or restored. The west door was unblocked, the church was re-floored, and the porch, vestry and organ chamber were re-roofed. Seating was removed, with chairs added, and the vestry heating chamber was altered. In 1913 there were two bells, from 1684 and 1902.

Sources include:
Cadw, Listed buildings Database
Cambria Archaeology, 2000, Carmarthenshire Churches, gazetteer, pp. 48
Richard Suggett, Painted Temples: Wallpaintings and Rood-screens in Welsh Churches, 1200–1800, (RCAHMW 2021)

RCAHMW 2022