In 1868 Clewer was described as a parish a mile from Windsor, but today it has been almost entirely subsumed by its neighbour. Clewer is at the northern extremity of Windsor, alongside the Thames, whose floodplain has effectively put a stop to expansion in that direction. St Andrew's is a flint church with a short W tower carrying a shingled broach spire. Otherwise it has a chancel with a S chapel, a nave with N and S aisles, the N continuing eastwards as a chapel, now used as an organ loft, and vestry, and a modern S porch. The present S aisle was the original, early 12thc. nave of the church, and its plain chancel arch now leads to the Brocas chapel; the old chancel rebuilt as a chantry by Sir Bernard Brocas in the 14thc. The present nave was added in the later 12thc., and the three-bay S arcade dates from that time. The present N arcade appears to be of the early 13thc., but there is a record of "a new N aisle with wall repairs" added by Henry Woodyer in 1861. Woodyer may thus have widened an existing aisle. The chancel was also restored by Henry Woodyer in 1868. The plain 12thc. chancel arch is round-headed, of one order with plain hollow-chamfered imposts. Romanesque sculpture is found on the S nave arcade and the font.

Clewer was held by Earl Harold before the Conquest, and by Ralph FitzSeifrid in 1086. It was assessed at 5 hides before 1066, but the Conqueror appropriated half a hide for his castle of Windsor. By c.1200 Richard de Sifrewast was the Lord of the Manor and it remained in the male line until 1441, when it was divided between three daughters. The advowson of the church remained with the manor until 1661.

VCH and Pevsner disagree about the N arcade; the former attributing it to Woodyer and the latter to the 1180s. The font is one of several arcaded examples in the county, e.g. Enborne, Avington, Radley and St Agnes Spital, Windsor but none are very close to this one. Pevsner, Hewitt and VCH agree on a twelfth century date, and the complexity of the foliage forms suggests a link with some of the Reading Abbey cloister work, e.g. the font in St James, Reading.