St Mary's is an impressive church that dominates the town centre. The church guide states it was built in 1438, whilst Pevsner gives the date of consecration as 1458. The church originally comprised a nave and chancel with piched roof and battlements, S and N aisles, and a fine S porch. It also has an attached three-stage tower. The whole building was constructed from large, rough granite blocks. The current church remains as a fine example of 15thc architecture, and both the mid 19thc restoration and the slightly later N aisle expansion have preserved this character.

Callington is a busy small town, rather than a village; the presence of an imposing Romanesque font indicates that its importance was already established in the 12thc.

The church contains a granite Altarnun-type font.

No church is mentioned in the Domesday Book. Callington Church was originally a chapelry to Southill; the present ambitious building dates from 1438, when Callington first obtained the right of burial.

The church fell into disrepair in the 19thc and was restored by J. P. St Aubyn in 1858/9. An outer N aisle was subsequently added with sensitivity by J. D. Sedding in 1882.

According to Henderson, documents show that there was an earlier chapel on the site in 1384, and the Romanesque font may be the only remnant from this structure. The font is presumably a relic of the original Norman chapel; Sedding dates it as between 1100 and 1130.

Sedding believed the stone to have come from Hicks' Grey Mill quarry in Lewannick.

Sedding pointed out that the geometrical patterns on the font 'in this case are cut with fillets, instead of the usual sharp arris, on the raised lines of the patterns, thereby giving it a separate place among others of its kind. The fillets are about a quarter of an inch wide'. The use of rosettes and 'Trees of Life' on different sides of the same font is unusual amongst Cornish churches.

This may be because the E and especially the S face appear to have been recut at a later date.