This church is not mentioned in the Domesday Book, unlike its larger neighbour, St Sampson, which was held by St. Peter of Westminster. St Mary's may have begun as the daughter church of St Sampson.
"The town lies within a square earthwork enclosure close to the point where the Roman road from Silchester to Cirencester crossed the Thames." In the 11thc. timberwork around this enclosure was replaced by a wall of mortared masonry. (Pevsner, 1975, 201)