Holy Trinity Church is situated in the centre of Cilcennin, The churchyard is depicted as curvilinear on historic (1889) Ordnance Survey mapping, but had been extended on its south-west side by 1905 and has now been extended further. The churchyard is bounded by roads to the south and east, and by the curtilage of modern buildings on its north side. The church is known to have existed in 1684. Cilcennin had become a parish by 1833. A well, Ffynnon-Fair, is depicted on historic (1889) Ordnace Survey mapping, immmediately east of the road, some 250m north of the church.

The 1684 church was depicted as a single-celled stucture with a plain, square-headed doorway, two plain windows and a west single bellcote.

The church was entirely rebuilt in the early nineteenth century (before 1833) on the same site and in the same location as its predecessor, but retaining nothing from the earlier fabric. It consisted of nave and chancel with tall, gabled single bellcote. The church was again substantially rebuilt in 1889-1891. The present church, constructed of local rubble stone, consists of three-bayed nave and chancel, west bellcote and west porch, transeptal vestry and bier-house (divided by internal walls). The church was reroofed, refloored and reseated at that time. The gabled single bellcote on the nave west wall dates from 1990-91. A possible medieval font bowl was noted, lying loose in the porch, in 1906.

Sources include:
Cambria Archaeology, 2000, Carmarthenshire Churches, gazeteer, 48
Ordnance Survey, 1889, first edition 25in
Ordnance survey, 1905, second edition 25in
Ordnance Survey, modern, 1:10,000

S. Clements, RCAHMW, 4 Septmeber 2013