Both St Thomas' and the nearby Monnow Bridge were damaged by fire in the Battle of Monmouth in 1233, part of the series of uprisings against Henry III by his barons.

[8] It appears to have been badly neglected by the early 19th century; in 1829, Bishop Huntingford's inspection referred to it as "this dilapidated and forsaken church".

A major restoration and extension of the church was completed by the London architect Thomas Henry Wyatt, who added box pews with raised galleries.

That on the south wall has a pillar base with scrollwork and a bowl with images of the faces of a man and a woman, pelicans and a serpent in a Garden of Eden theme.

[10] This dating echoes E. T. Davies, who suggests, in his 1977 booklet, The Ancient Churches of Gwent, that the styling of the pillar indicates that the font "goes back to the end of the eleventh century".

[2] Cadw follows Newman, describing the font as having a "19th century interlace stem and [a] charming, possibly recut, Romanesque bowl".

The church sits on land between the road and the river Monnow and a contemporary ceramic mosaic has been installed by Monmouth Town Council.