St Cynog's Church is situated within a circular churchyard, whose south, east and north boundary is delineated by the adjacent road. A crop mark identified by Dyfed Archaeological Trust in the field to the south-west of the churchyard is thought to be part of a concentric outer enclosure. An iron age enclosure is noted immediately further to the west, and the sites may be associated. In the medieval period St Cynnog's was a chapelry of Llanstephan beonging to the Deanery of Carmarthen. By 1833 Llangynog was a parish.

The building is consructed of rubblestone and consists of two-bayed chancel, three-bayed nave, south chapel (known as the 'Coomb Chapel'), four-bayed south aisle, south porch, vestry (north of chancel west bay) and coal cellar (north of nave). The nave is thought to date to the thirteenth century, and has a blocked west door. The chancel is thought to date to the fourteenth century. The south aisle and chapel are thought to date to the seventeenth century, although the chapel's eastern half may possibly be a later extension. The octagonal font was presented by Miss Lloyd of Laques in 1850. The nave and aisle were re-fenestrated in the mid-nineteenth century. The church was restored in 1876-1878, to the designs of architect George Morgan. The porch and vestry were added at this time, and the chancel was re-fenestrated. The chapel arch and bellcote were rebuilt and the church was re-roofed, re-floored, re-seated and re-plastered. The south door may have been rebuilt, and a mural on the chapel walls, consisting of painted tiles and mosaic infill, also dates to this time. A second font, decorated with carved foliage, dates to 1902. The coal cellar was added in the early-mid twentieth century.

Sources include:
Cambria Archaeology, 2000, Carmarthenshire Churches, gazetteer, 48
Lloyd, T, Orbach, J and Scourfield, R, 2006, The Buildings of Wales: Carmarthenshire and Ceredigion

N Vousden, RCAHMW, 14 December 2012