St. John's stands close to Devizes Castle and may have begun as its chapel. The oldest parts of the building are from 1130, shortly after Roger, Bishop of Salisbury rebuilt the castle. Pevsner writes "A major Norman church, dominated by a mighty crossing tower ...". The western part of the church was rebuilt in the 15th century. restoration was carried out in 1844 and 1862–3, including the west front designed by Slater. The ornate Beauchamp south chapel is similar to the 1492 Beauchamp and Tocotes chapel at Bromham; the north Lamb chapel has a fine panelled ceiling. The organ case is late 17th century.