Harmondsworth is on the W edge of the traditional county, under a mile from the Buckinghamshire border. It is now in the S of the London Borough of Hillingdon. It retains some of its village character despite being squeezed between Heathrow airport to the S, the M25 to the W and the M4 to the N.

St Mary’s stands at the N end of the village, E of the celebrated late-medieval tithe barn. The church consists of a chancel with N aisle, nave with N and S aisles and a S porch, and a tower at the W end of the S aisle, alongside the porch. A 19thc vestry towards the W end of the N aisle now contains lavatories and is linked to a later 20thc flat-roofed parish room; extremely unprepossessing from the exterior but providing a large and surprisingly airy space for functions.

In detail, the three-bay S arcade of the nave, and the reset S doorway are the oldest parts, dating from the 12thc. The S aisle is shorter than the N, owing to the presence of the tower at its west end. The ground storey of the tower was formerly a baptistery and still contains an octagonal Purbeck font; originally 13thc but with a shiny new bowl. The 13thc N arcade is of three bays too, but the E bay has been partly replaced (see below). The chancel was rebuilt in 1396-98, after the church passed to Winchester College, and has a small S doorway and a three-bay N aisle with a four-centred arcade of c.1500 of which the two E bays form a Lady Chapel, and the third bay houses the organ. The chancel aisle is contiguous with the N nave aisle, and there was clearly an intention to extend the four-centred arcade of the chancel westwards to replace the older nave arcade, but this only reached the apex of nave bay 1. The upper storeys of the tower are of brick and date from c.1500; the tower is capped by a small cast-iron cupola. The S porch is 19thc. Construction is of flint with a good deal of ashlar rubble incorporated. Romanesque sculpture is found on the S doorway and the S nave arcade.

Before 1066 the manor of Harmondsworth was held by Earl Harold, and it became William I’s at the Conquest. In 1069 he gave it to the Benedictine Abbey of Holy Trinity, Rouen, along with the church. A cell was founded at some time after 1086, probably in the 12thc, and a prior is first mentioned in 1211. According to VCH, the prior was effectively the Lord of the Manor; a situation that lasted until 1391, when the priory was dissolved and its lands, along with the manor, passed to Winchester College.

Pevsner (1951) describes the S doorway as the most elaborate piece of Norman architecture in the county next to Harlington, which shares features with this doorway including beakhead (but of a different kind), similar chevron and similarly ornamented nook-shafts. Harlington is more elaborate but probably a product of the same workshop. Such features suggest a date in the 1130s, both earlier than the nave arcade and aesthetically very different. Pevsner suggests that the arches, being pointed, must date from after 1200. For the present author a date in the 1160s or possibly the ‘70s is acceptable for the entire arcade, especially in view of the suggestion of trumpet scallops in the capitals. This leaves the font, omitted here but dated c.1200 by Pevsner and the EH listing. While the supports might be 13thc, the bowl is surely a modern replacement, especially as it has no signs of any locks or staples.