St Cynin's Church was a medieval chapelry of St clears in the Deanery od Carmarthen, and was a posession of the Cluniacs of St Clears Priory (NPRN 275712) Llangynin was a parish by 1833. Modern and historic(1889) Ordnance Survey mapping depicts a footpath which branches south from a minor road (beginning at the point the road turns south-west) some 85m south of Blaenffynhonnau farm, and continues in a south-south-easterly direction until it arrives at the east churchyard boundary. One of the church's south door jambs is reportedly derived from an inscribed stone, but no inscription is now visible.

The church is a Grade II* listed building, constructed of limestone rubble. It consists of two-bayed chancel, three-bayed nave, north transept, three-bayed south aisle and three-storeyed west tower. The octagonal font is thought to possibly be medieval. The nave is thought to date to the late thirteenth or early fourteenth century. The chancel may date from the same time, and formerly had a fouteenth century window. The vaulted north transept was added in the fourteenth century, and formerly posessed a skew passage. Evidence for a southern skew passage suggests that there was formerly also a south transept. The south aisle dates to around 1500. The tower dates to the early sixteenth century, and a square spiral stair turret projects from its north wall. It connected with the nave via a low doorway with four-centred surround, and is lit by simple loops, some blocked with cement. The second storey was lit by openings in three of its faces, all of which are blocked. The belfry stage was lit by a two-light opening in its east wall and a single light opening in its north wall, both of which were blocked in the twentieth century. There were two bells in 1552, but only one by 1684. A seventeenth-eighteenth century alter table was reportedly in the church in 1974, but has now gone. The present alter table dates from the late twentieth century. The bell present in 1887 was recast in 1810. The church is thought to have been restored around 1920, when the chancel arch and two bays of the south aisle arcade were rebuilt. A new window was added to the nave north wall, the east window and north transept window were replaced. The chancel side windows and south aisle windows were also replaced but may be copies of the originals. A statuette noted to be lying loose in the south aisle stoup is thought to have been acquired from the restoration St Peter's Church, Carmarthen, and is reportedly a fifteenth century weeper from the tomb of Rhys ap Thomas.

Sources include:
Cambria Archaeology, 2000, Carmarthenshire Churches, gazetteer, 48
Ordnance Survey, 1889, first edition 25in

N Vousden, RCAHMW, 10 December 2012