The church comprises chancel, nave with N aisle, S transept and S porch, and W tower. Originally 12thc, the transept, tower, and part of the N arcade were rebuilt in the 14thc. The remainder was rebuilt in the 15thc and early 16thc. The vestry was probably added during a restoration of 1876 (Historic England listing: 1162910; Pevsner 1989, 803). The elaborate S doorway is the only feature surviving from the 12thc.

In 1066 the abbey of St Mary and St Rumon in Tavistock held Thornbury. At the time of the Domesday Survey it was held by Ralph with the abbey as tenant-in-chief.

There are similarities to a number of more elaborate doorways found in Devon and Cornwall (Buckland Brewer, Morwenstowe, Parkham, Shebbear and West Woolfardisworthy in Devon, and Kilkhampton in Cornwall. A fragment of label, of the same type as described above is reset into the porch at Molland, suggesting that a similar doorway may once have existed there.