St Mary's has a nave with three-bay aisles and a clerestorey. The two E bays of the N arcade are round-headed with scallop capitals and chevron on the arches, c.1150-70. The corresponding bays on the S are slightly later, with waterleaf and chamfered arch orders. The third bay on each side is an addition of c.1225. The tower stands at the E end of the S aisle, the bay below it now housing the organ. It is later 12thc. in its lower parts, with a simple S doorway and a plain window above. The S nave doorway is late 12thc. and stands under a porch. The N porch has been blocked and converted into a vestry. The chancel is described by Pevsner as 'an over-restoration of 1844.' Romanesque sculpture is found in the E bays of both arcades, the two S doorways and the S tower window.

One hide at Duddington belonged to the royal manor of Gretton in 1086, and a priest was recorded on this land (although RCHME Report, uncatalogued suggests that he may have been the priest of Gretton). The chapel at Duddington was mentioned in a bull of Eugenius III to Lincoln Cathedral in 1146. In 1246 the church was possessed by the vicar of Gretton.

Waterleaf capitals similar to those in the S arcade are found at nearby Warmington. The arrangement of the doorway is most unusual; it may be a reset porch entrance.