St Mary's Church is situated at the junction of Gray's Inn Road and George Street. It is a small Victorian church built in the early Decorated style, established by Rev. E.O. Phillips (later Dean of St Davids) in 1863-1866, to serve Welsh-speaking members of the established church, on land donated by Colonel Powell of Nanteos (NPRN 278). It was built to designs of William Butterfield, who abandoned the job after quarrelling severely with the Rev. Phillips.
The church is constructed of rubble masonry with freestone dressings, banding and gable parapets. It consists of a five-bay clerestoried nave with aisles, west double-bellcote and gabled north porch, and two-bay chancel. The north aisle overlaps the chancel to incorporate the vestry.
The interior is whitewashed on simple plaster, even to the arcades, accentuating the focus to the east. The nave roof is considered architecturally unusual, emphasizing broken lines rather than smooth curves, with sides in two pitches and schematic wind bracing. Fittings include a stone reredos with painted circular panels, a panelled timber pulpit, and an octagonal font on a quatrefoil shaft gilded and painted. . The nave east window has stained glass by Heaton, Butler and Bayne (1861), removed from the former St Michael's Church (NPRN 306590) by John Davies in 1892. The glass in the north aisle is by H. Wilkinson (1925) and A.O. Hemming (1903). The organ (of 1840), in a Gothic case, is also from St Michael's church.
Sources include:
Cadw Listed Buildings Database
Lloyd, T., Orbach, J., Scourfield, R. 2006. The Buildings of Wales: Carmarthenshire and Ceredigion.

D Leighton & N Vousden, RCAHMW, 3 July 2015