St Ystyffan Church was granted to the Knights Hospitallers of Slebech around 1170, by Geoffrey Marmion. The grant included a fishery and ferry across the afon Taf, and was confirmed in 1231-47. In 1290 the church was re-granted to the Prior of the knights Hospitaller in England. The grant was confirmed in 1328. After the dissolution the advowson fell to the Crown. It subsequently found its way into private patronage, and by 1833 the living was a perpetual curacy in the joint patronage of William Lloyd Esq. of Laques Mansion, Llansteffan, and a Carmarthen banking firm. The church's side chapel belonged to the Lloyds of the Plas and nearby Laques and contains memorials. There are also memorials to the Parnall family. Historic Ordnance Survey mapping depicts a sub-circular pound abutting the north-east churchyard boundary.

The building is constructed of limestone rubble. It consists of nave with three bays, chancel with two bays, north transept, south transept, north chapel with three bays, south porch over below-ground boiler house and west tower with three storeys. The nave dates to the twelfth to thirteenth century. The west bay of the chancel and the south porch both date to the fourteenth century. The plain, two-centred south door is also thought to date to that time. A piscina in the south wall may be fifteenth century. The plain, square squint is fifteenth century, and the transepts also date to this time. The tower dates to the early sixteenth century. A square spiral stair turret projects from the east half of the north wall. It is lit by simple square slits. The two-centred west door is sixteenth century, with a plain three-light window above from 1869. A blocked semi-circular headed window is thought to date to the twelfth-thirteenth century. The second storey east wall has a small, square single-light window from 1872-1875, a simple sixteenth century slit-light, and a rebuilt sixteenth century lancet with a four-centred head. Its north wall has a cusped lancet from 1872-1875, in an earlier opening. The tower's other two walls have sixteenth century slit-lights, with an additional window in its west wall. The belfry stage has sixteenth century segmental single-light openings in all four walls, with two-centred surrounds which were inserted in 1872-1875. There were three bells in 1552, but only one in 1672. The east chancel window dates to the sixteenth century and comprises two uncusped lights in a square surround. All nave window openings are from 1829-1830, but the two-light windows with square surrounds date to 1872-1875. The boiler house was inserted at this date, as were the stalls and pews. The peal of eight bells also dates to 1875. The organ dates to 1887. The church was renovated in the 1920s and new heating apparatus was installed in the boiler house. The tombs in the north chapel were lowered to be flush with the floor in 1983. Further renovations were undertaken in 1995-1997, when all the roofs were re-slated and the tower was re-pointed, with some of its openings being rebuilt. The plain, two-centred south door is modern, and the blocking of the arch from the north transept to the chapel is also thought to be modern.

Traces of wall paintings were visible in the transepts in 1975. They consisted of three superimposed layers of wall painting, each in fragmentary condition. On the west wall of the south transept was the remains of a Welsh apostles? Creed, and a black letter inside a yellow and black cabled border. Below the creed two earlier layers were visible, thought to be pre-reformation in date. On the west wall of the north transept was a painted harp and the remains of a picture of a larger harp, with only the strings and hand surviving.

Sources include:
Cambria Archaeology, 2000, Carmarthenshire Churches, gazetteer, 48

N Vousden, RCAHMW, 22 October 2012