St Teilo's Church is situated within a large, curvilinear churchyard, and is a conspicuous feature in the town. The churchyard is encircled on all sides by roads, and bisected by the A483. The pre-conquest Lichfield gospels mentions an ecclesiastical establishment at Llandeilo Fawr. The church was a parish church during the post-conquest period, belonging to the Deanery of Stradtowy. The church may originally have been a possession of the bishops of Llandaff, but had passed to St Davids by the twelfth century. It was appropriated by Talley Abbey (NPRN 92750) around 1215, but the grant was disputed. This resulted in the abbot and convent being ordered to pay 10 marks a year for Llandeilo fawr and Llanegwad churches. There were a number of chapels of ease within the parish. Two tenth-eleventh century knotwork decorated cross-heads are located within the church, in the north aisle. One was discovered beneath the chancel during the 1850s restoration, and the other had been discovered by 1893. A Latin-inscribed stone stood in the churchyard in 1697, but was lost by 1893. The church's octagonal font dates to the fifteenth century and is from St Tyfei's Church, Llandyfeisant (NPRN 114230). A spring is located within the churchyard. This is covered over and culverted, and exits through the churchyard wall at road level, into a vaulted well-chamber.

The medieval church comprised a double nave and fifteenth century west tower. In 1845 the north aisle may have been roofless. In 1848 the body of the church was demolished, and rebuilding began, with stone excavated from a quarry within the churchyard. The quarry reportedly caved in, killing one of the workers.

The present church is a Grade II listed building. It consists of seven-bayed nave and chancel, south transept, six-bayed north aisle (whose east bay lies above a vault), north porch, vestry (south of the east bay) and transeptal organ chamber (between vestry and south transept). The tower is thought to date to around 1600 and is similar to the contemporary tower at St Tybie's Church, Llandybie (NPRN 100804). The church was largely rebuilt in the Decorated style in 1848-51 by G.G. Scott following a competition (drawings survive in the church). The four-stage tower is constructed of squared, coursed rubble with dressed quoins and a splayed base below the stringcourse. It has a nineteenth century pointed door cut into the west side and a two-light cusped Perpendicular window with returned label on the north side on the second stage. There are three large clock faces to the third stage, and two-light cusped Perpendicular windows to the bell stage openings on each face. There is a stringcourse below the nineteenth century crenellated battlements with gargoyle waterspouts. A stair turret to the north-east angle rises above the tower, lit by slit-like vice windows.There are three-light decorated windows to the six-buttressed bay on the north side, having snecked rubble masonry ashlar dressings. The gabled porch is in the second bay from west, with a pointed doorway and nookshafts. There are similar four and five light windows to the west and east ends respectively. The slate roofs are steeply pitched, with raking gable parapets on kneelers. The chancel is advanced by one bay, with a small three-light window to the north, large six-light east windows, and angle buttresses. An outshot vestry has two-light windows, cusped and Caernarvon arched. Tthe twin gabled south transept has angle buttresses, three and two-light windows. The four nave windows, and the buttresses, are similar to those on the north side. Inside the church octagonal arcade piers rise to plain arches with hood moulds. The north nave is subdivided by modern partitions creating a corridor to the south nave with service rooms to the north and south ends. There is a small tablet to the Reverend W Owen, 1791.

The south transept has a black marble tablet to John Griffiths of Glancennen, died 1754, along with other monuments. There is a marble relief of a classical mourning figure with lamp on a pedestal, to Cecily, Baroness Dynevor, died 1793, by J Bacon of London. A similar figure relief with an urn is for George Rice of Newton House, died 1770. There is also a classical architrave in marble with triglyph frieze, gadrooned base and bracket, to Catherine Rice of Newton House, died 1717, by John Rendall of Bristol.
A bronze relief of W Phillipps who died in 1908 was made by Elkingtons of London.

RCAHMW, 10 February 2009