Bletchley is now part of Milton Keynes, but in 1967 when Milton Keynes was designated it was already a town, one of three that were to be encompassed by the "new city". It is in the SW of the Milton Keynes designated area. Bletchley is best known for Bletchley Park, the World War II codebreaking centre, and the church stands on the southern edge of the grounds of the mansion.

It consists of a chancel with a N chapel, an aisled nave with a S porch, and a W tower. Most of the fabric is 13thc or later: the nave and chancel are of this date while the S nave aisle was added c.1300 and the N aisle, chapel and porch are 14thc work. The tower is 15thc. The arch of the S doorway, however, contains 12thc voussoirs reused in the 1300 wall. This is the only Romanesque feature.

Bletchley was not mentioned by name in the Domesday Survey, but later contained 2 manors, included in 1086 in Water Eaton and Great Brickhill respectively. The church was in the Great Brickhill manor, attached to the honour of Giffard or Gloucester. In the early 13thc the manor was held by John Grey, Lord of Water Eaton, remaining in the same family until it was sold to Katherine, dowager Duchess of Buckingham, in 1630. The tomb of one of the Greys is set on the N side of the chancel. The church was built before 1212, at which date Gerald de Caux was its parson. The advowson was held by Roger de Caux in the episcopate of Hugh of Wells (1209-35).

The beakhead and cusped arch above the S doorway is the only evidence for the church at Bletchley before the 13thc. It suggests a highly decorated, mid-12thc building. It appears to have been incorporated when this section of the church was added c.1300. It has been suggested that the arch may have come from a doorway or the chancel arch, and the large diameter and good state of preservation both tend to support the latter theory (see List Description). The presence of a chevron voussoir in the arch suggests that there was another decorated arch somewhere in the 12thc building - presumably a doorway.

Beakhead is found in at least four other Buckinghamshire churches, although in two cases it is a geometrical form of the ornament (beaker clasp or proto-beakhead). The richest and most varied work of this type is on the reset chancel arch from Stantonbury, now at Little Bradwell just 7 miles to the N on the other side of Milton Keynes.