All Saints' has an aisled and clerestoreyed nave with four-bay arcades of c.1300 and a Perpendicular clerestorey. The S doorway, reset under a porch, is 12thc., but the N doorway is 13thc. The chancel is also 12thc., with an original priest's doorway, although Perpendicular windows were added on the south. The chancel arch is 12thc. but remade with a pointed arch c.1300. On the N of the chancel is a chapel of the late 13thc., now housing the organ, and there is a vestry to the E of this. The W tower is 13thc. except for a later parapet. The tower is of rubble; the remainder of ashlar. Romanesque features are the S nave and chancel doorways, the chancel arch, and an elaborately carved font.

Harpole is a rare example of an estate held by the same man before and after the Conquest. This is Bisceop, who held it freely in the Confessor's time, but from William Peverel in 1086. A priest was recorded at that time. Harpole remained a rectory with the advowson in the hands of the lord of the manor throughout the Middle Ages, although some tithes were granted to St Alban's abbey. Disputes with St Alban's, presumably connected with these tithes, are recorded throughout the 14thc.

Benefice of Kislingbury and Harpole.

Pevsner describes the font decoration as "'a remarkable tangle'. In fact the font and the S nave doorway outer capitals are products of the same lively workshop, perhaps active in the 1140s or '50s. The doorway also has scallop capital similar to those of the chancel arch, which suggest that this is contemporary too, but the waterleaf capitals of the chancel doorway point to a date at least thirty years later.