There has been a church on the site of St Mary’s since Saxon times. By the 10th century the area of the Thames Valley over which Buckland looks was wealthy and home to a relatively large population. There was a royal manor at Bampton, and there were many important minster churches (monastery churches that acted as small scale cathedrals) in the area, particularly at Abingdon, Faringdon, Bampton, and Eynsham. It is possible that some of the west wall dates to this time.
Most of the nave dates to the 12th century, when the well-connected de Bucklands held the manor. The transepts were added shortly afterwards in the 13th century, as was the porch and tower. The chancel was enlarged in the 14th century and later restored in the 19th century.
The church has a cruciform (cross-shaped) plan with a tower at the centre of the cross. This is in common with many early medieval churches in the area, including those at Bampton, Witney, Minster Lovell, Burford, Faringdon, Uffington, and Wantage. This and its large chancel and transepts (all areas that would have been screened off from the public and kept for the use of the clergy) imply that the church had more than a single parish priest, and could even have had a small college of priests.