The church has a nave with a north aisle and vestry, a Perpendicular Gothic west tower, a chancel and a south porch. The nave was rebuilt in the late 15th or early 16th century to meet a tower of approximately the same age; it is often dated to 1521, but appears to be built on older foundations. The southern side of the nave contains what are probably thirteenth-century buttresses and a pair of Perpendicular Gothic windows. The north aisle was originally built in the 13th century, and rebuilt by H.J. Underwood in 1890;[dubious – discuss] the vestry was built in the 17th century and rebuilt in 1846 to designs by Chamberlain, through the generosity of the curate, Alexander Penrose Forbes. The church has been reroofed at least twice, in 1825 and 1897.
The chancel, which has a ceiling decorated by C. E. Kempe, has three windows in the style of the late 12th century, and a priest's door built into the south side circa 1250. A south porch was built in 1621 at the behest of Dr Robert Burton, whose arms are carved in the gable above the date. A candelabrum given by Ann Kendall in 1705 hangs in the chancel. The chancel ceiling was decorated with a pattern of gold stars on a blue background in 1914. Two years later, an altar was erected at the east end of the north aisle, and an aumbry placed in the north wall of the chancel. The royal arms of William IV are displayed in the tower.
St Thomas' church has been a Grade II listed building since 1954.