St Hilary's Church is situated within a triangular-shaped churchyard, with two of its boundaries delineated by roads. The third boundary is depicted as curvilinear on historic (1888 and 1905) Ordnance Survey mapping but is now straight. The church is some 20m north-east of a motte and former bailey, Castell Trefilan (NPRN 303879). It has been noted that the grouping of castle mound and church is suggestive of a llys site. The church was a parish church during the medieval period, belonging to the Deanery of Ultra-Aeron. It was mentioned in documents of 1282 and is thought to possibly be contemporary with the foundation of a town at Trefilan in the thirteenth century. The church's medieval dedication is thought to have been to St Mary. Its 'Hilary' dedication has been sugested to derive from the place-name, 'Ilan' becoming confused with 'Ilar', the Welsh form of 'Hilary'. In 1833 the living was a discharged rectory in the patronage of the Bishop of St Davids.

The pre-1806 church is thought to have been larger than its successor. The cylindrical font stem and square base (now lying loose in the church) are late twelfth- early thirteenth century in date. It was demolished and entirely rebuilt in 1806, on the same location as its predecessor. The medieval font was retained, but the font bowl had disappeared by 1914. The 1806 church comprised three-bayed nave and chancel and west bellcote. It had two small, two-centred neo-gothic windows in its south wall and one in its east wall. These were blocked sometime before 1880 and replaced with three larger windows with timber Y-tracery. A west porch is thought to have been added at the same time.

The current church, constructed of local rubble stone, was almost entirely rebuilt in 1880-1882, to the designs of A. Ritchie, Chester. It consists of four-bayed nave, two-bayed chancel, south porch, vestry and eastern bellcote. The core of the east and south walls are from the 1806 church. The current north and west walls extend considerably further than those of 1806. The south porch east wall was rebuilt in 1987.

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

N Vousden, RCAHMW, 27 February 2014