The present church consists of a nave with S aisle and W tower, and an aisleless chancel (with chancel arch) terminating in an apse (with apse arch). There is some herringbone masonry in the nave N wall, suggesting an 11thc. date for the earliest work. The church was used as a Parliamentary barracks during the siege of 1646, and was left in a ruinous state with only parts of the chancel and N wall still standing. There were repairs in 1656, 1695 and 1700, but the church owes its present appearance to Hakewill's restoration of 1849-50, which included the rebuilding of the apse, and the replacement of the S aisle, the W tower and the font.

Despite its heavily restored state, St Leonard's contains important 11thc. work in the chip-carved chancel and apse arches. The N doorway appears to be entirely 19thc., but Pevsner (1966) calls it Norman, and it is also described below.

Wallingford was destroyed by the Danes in 1006 (AS Chron.). In 1086 it was the chief town of Berkshire, with a castle, several churches, a market and a moneyer. No information has been found which would suggest a date or a patron for the 11thc. building. The church was presented by Henry I to the monastery of St Frideswide, Oxford, in whose possession it remained until the Dissolution.

Tyack et. al. (2012) criticize Hakewill's work as uninspired, but fint the 11thc apse and chancel arches splendid, dating them to the 12thc. The list description places the chancel arch, the apse arch and the doorway in the 19thc.