St Mary's has an aisled and clerestoreyed nave with four-bay arcades. In each arcade the two western bays are 12thc., and the two eastern bays 14thc. The short 12thc. nave seems to have been lengthened eastwards in the 14thc., and the clerestorey was added at that time. There are two doorways: the 12thc. S doorway is elaborate and protected by a porch; the 13thc. N doorway very plain and unprotected. The chancel and its arch are also 14thc. The W tower is 15thc. (money was left for the fabric of the 'campanile' in 1453) and of five storeys, the two lowest with ashlar bocks in alternately brown ironstone and grey limestone courses. Above this the ashlar is newer and appears 19thc. The clock is dated 1862. The nave, aisles and chancel are faced in stone rubble. Romanesque sculpture is found in the W bays of both arcades and the S doorway.

Grendon was held by Countess Judith in 1086. Three mills were recorded but no church. The countess granted the manor to King David of Scotland as part of a knight's fee, and his successor Malcolm IV granted the church to the Abbot of Jedburgh (along with Brandon, Northants). A dispute between the abbot and the Earl of Huntingdon over the right of presentation to the benefice was resolved in 1231 in favour of the earl.

Benefice of Yardley Hastings, Denton and Grendon with Castle Ashby and Whiston.

The use of circular imposts in the arcades is unusual, but also found in the Holy Sepulchre, Northampton. Pevsner (1973) calls the arcades Late Norman, and the doorway (which he implies is later) late 12thc. He does not suggest, with the present author, that the doorway is of two periods, but this seems beyond dispute. The use of nailhead and half-daisies in both the arch of the doorway and the capitals of the arcades suggest that they belong to the same campaign, which the present author would place in the middle of the 12thc. rather than the end.