A cruciform church with an aisled nave and a W tower with chapels to N and S (i.e. a species of W transept). Most of this, that is to say the lower stage of the tower with the W doorway and the N, S and E arches inside, the N nave arcade, the clerestorey with circular windows with plate-tracery quatrefoils, the S nave doorway and the N transept must date from the early 13thc. The S nave arcade, however, is older in conception and is described below. The S transept is in fact a later chapel, and looks 14thc., the same date as the upper storeys of the tower. The aisleless chancel is presumably c.1200 but its windows are later and the chancel arch is 19thc. Construction is all of pebble rubble with ashlar dressings, and the tower has a crumpled spire.

In 1086 Bourn was substantially held by Picot of Cambridge. The manor was a large one, assessed at 13 hides, which Picot had received as two manors originally. The description of this core holding mentions two priests. In addition, the Abbot of Ramsey held 1 hide at Bourn as a berewick (outlying estate) of the manor of Longstowe, which was part of the abbey demesne. A further 4 hides were held by Count Alan and by Almaer from him. Before the Conquest, the same Almaer had held the land from Eadgifu. Finally Peter de Valognes, Sheriff of Essex held 1 hide and 3 virgates and Picot held that from him. To confuse matters further, this land had also been held by Almaer before the Conquest. The church at Bourn was given by Picot to Barnwell Priory when he founded it in 1092.

A similar arcade with octagonal piers and multi-scallop capitals may be seen at St Andrew's, Soham.