Ashendon is in the W of central Buckinghamshire, seven miles W of Aylesbury. This small village is on a limestone hill on one of the network of minor roads in this area of pasture on the Kimeridge clay, and the church is in the centre of the village. St Mary's consists of a nave with a S aisle and S porch, a chancel and a W tower. The church is of limestone rubble with patches of herringbone masonry in the N wall of the nave, where there is also a completely plain, round-arched doorway. At the E end of this N wall are signs of the former Falconers chapel in the form of a blocked arch and a roofline visible on the exterior. A date of 1554 is associated with this, but it is probably 14thc. in origin to judge from the arch profile and the W capital. The E capital is castellated and may reflect a 16thc. modification of the chapel. There are the remains of the rood-loft to be seen on both sides of the nave, and a squint from the S aisle into the nave at the E end. The nave was lengthened westwards at the beginning of the 13thc., for which a single plain lancet provides the evidence. The nave roof was heightened in the 15thc., when a clerestorey was installed. The S arcade is in two parts. At the W end is a single 12thc. round-headed bay cut through the wall, with broad rectangular piers to the E and W and the arch supported on imposts. The two eastern bays are 14thc., with moulded capitals, double-chamfered arch and an octagonal pier. The chancel is 19thc., and the low, two-storey W tower is 15thc., with diagonal buttresses and Perpendicular W doorway and window. The only Romanesque sculpture here is on the font.

The manor of Ashendon was held by Walter Giffard in 1086, and by Richard from him. The overlordship descended with the honour of Giffard until between 1247 and 1255, when it passed to William de Valence on his marriage to Joan de Monchensy. In the early 14thc. it passed to the Argentein family, in which it descended throughout the middle ages. Nothing more is known about Richard, the Domesday tenant. By 1213 it was in the hands of Thomas, who was succeeded by Henry St Andrew in 1236, and it remained in this family until the early 14thc. The church was given by Walter Giffard to Nutley Abbey and remained in its possession until the Dissolution. It is now part of the Bernwode benefice, i.e. Ashendon, Boarstall, Brill, Chilton, Dorton, Ludgershall and Wotton Underwood.

This is a form of font common in Buckinghamshire, with a tub or cup-shaped bowl on a thick roll supported by a stem. Other examples are at Haddenham, Newton Longville, Wingrave and Bierton, and the Aylesbury group are an elaboration of the basic design. VCH describes the font as 12thc. but retooled at a later period.