Mellor stands in the High Peak on the border with Derbyshire. Indeed it was in Derbyshire until 1936 when it was reassigned, along with neighbouring Ludworth, to Cheshire. Recent excavations have disclosed an Iron Age hill fort alongside the church. St Thomas's was formerly a chapel of ease to Glossop in Derbyshire, and remains in the Glossop Deanery of the Diocese of Derby. The church has a 15thc. W tower, but whatever was to the east of this was replaced from 1815 to 1830 with a simple aisleless nave and chancel of brick. Something similar took place at Church Lawton. The only Romanesque feature is the font, one of the most interesting in the county.

Formerly chapel of ease to Glossop.

Mellor is not recorded by the Domesday Survey, and the church was apparently not noted before it appeared on a 1589 map of the parish. of Glossop.

A similar incised technique with large beasts appears on the fonts at Tissington and Thorpe (both Derbyshire), but Mellor's font is more adeptly and rhythmically carved. It is certainly an 11thc. piece, but whether pre- or post-Conquest is uncertain. Bond (1908) included Mellor in a list of fonts 'which have been credited with undue antiquity, owing to the rudeness or uncouthness of their ornament; but it by no means follows that what is archaic is always ancient.'