Originally a chapel of ease to its mother church of St Stephen, St Thomas's church lies midway between St Stephen's and Launceston. The church is built predominantly in the Perpendicular style. The Romanesque features of the church are a large font, a tympanum and a crude figure panel.

The date of the foundation of the church is unknown because it is not mentioned in the Domesday Survey but Henderson states that it "witnessed both the rise and fall of its great neighbour", the Priory. The chapel may have thus pre-dated both the new Priory (1126) and Launceston Castle chapel (1136). The cemetery was consecrated by Bishop Grandisson on November 6, 1333 'in Prioratu de Launceston'. Sometime afterwards the church became parochial, which it remains, the parish being within the borough of Launceston.

A drawing of 1958 inside the church shows the font, without the modern granite step, nearer to the W column of the S arcade. It is said by Pevsner to be the largest in Cornwall and is certainly the largest of its type. It seems certain that it came from the adjacent Augustinian priory's conventual church, especially as the parishioners of St Thomas's had been enjoined, prior to the Reformation, to use the baptismal font in the priory (3.). Possibly, the tympanum also came from the priory which was moved here from St Stephen's in 1126, and of which only the foundations remain, adjoining St Thomas's churchyard. Sedding described the tympanum as being placed over the S doorway and cut to suit its position. He also mentioned "two Norman stones" in the churchyard, which are no longer to be found.