St Tysilio's Church is situated within a curvilinear enclosure. The church was mentioned in documents of 1291, when the patronage was in the hands of the Bishop of St Davids (which may have continued an earlier association). It has been suggested that the churchyard may occupy the site of an iron age enclosure. The churchyard may have lain within a very large (c.850m diameter) outer enclosure, currently defined by areas of glebe land and field boundaries. Three inscribed stones, Llandysilio 1,2 and 3 (NPRNs 422416, 421733 and 422417), and one cross-carved stone, Llandysilio 4 (NPRN 422418), are built into the fabric of the church. A further inscribed stone (NPRN 275683) was moved here from St Michael's Church, Llandre-Egremont (NPRN 310025) around 1964 and was set into the floor at the east end of the nave. During the post-Conquest period the church was a parish church in the Deanery of Carmarthen. In 1536 it was a vicarage in the patronage of the Bishops of St Davids, the rector being the prebendery of Llandysilio in the Collegiate church of Brecon. In 1749 the living was sequested owing to a vacancy, and by 1833 the church was a discharged vicarage of the Archdeaconry of Carmarthen, still in the patronage of the Bishops of St Davids.

The church is a Grade II listed building, constructed of limestone rubble. It consists of 2-bayed chancel, 4-bayed nave, 2-bayed vestry (north of the chancel) and south porch. The nave and chancel may be 13th century. The chancel plain, 2-centred chancel arch is considered to be broadly medieval. The chancel south wall has two 16th century windows. The chapel and north aisle are thought to have been constructed around 1500. The church was restored in 1838, when it was partially rebuilt, with masonry from the medieval fabric said to have been reused in the construction of the house of the incumbent responsible for the rebuild. It was at this time that the early medieval inscribed and incised stones were incorporated into the external south wall. At this time the nave was widened to absorb the former north aisle, and a new north wall was constructed. The former north chapel was converted to a vestry. The windows were replaced by timber-framed sash windows. The church was again restored in 1896?1899, to the designs of Protheroe and Philpot, Cheltenham. The two 16th century windows in the south wall were reopened and the medieval piscine was exposed and retained. The chancel was reroofed and a collar-rafter gabled roof added to the vestry. A second bay was added to the vestry and the arcade leading to it was reopened and replaced shortly afterwards in a second phase of restoration commencing in 1898. All fitting s were replaced at this time, and an entirely new roof was added. The bellcote was rebuilt and the south porch was added. The west door was blocked and a new east door inserted. New, 2- and 3-light neo-perpendicular square-headed windows were inserted, and a new west window was built.

Sources include:
Dyfed Archaeological Trust, 2000, Historic Churches Project, Pembrokeshire gazetteer
Dyfed Archaeological Trust, 2003, Early Medieval Ecclesiastical Sites Project, Pembrokeshire gazetteer

N Vousden, 3 January 2018