The original Parish of Barnham took its name from 'Beorna' whose 'hamm' or enclosure was in this low-lying country. The Church was recorded in the 1086, but no history is known before that date. Originally the village was clustered around the Church, and up to the Middle Ages a tidal creek ran to within a few yards of the building. Fishing, not farming, was the main occupation of the villagers, as was smuggling, and there is said to be a secret underground passageway leading to the church. The modern village of Barnham has spread northwards, nearer to the Railway Station, and this has left the Church in comparative isolation surrounded by country and farmland.

Although mentioned in the Domesday Book, excavations have revealed artefacts of Saxon origin, and it is thought that originally a Saxon Church could have stood on the site. Parts of the Church are characteristically early Norman, the Font being of the same period. Unfortunately this has suffered severely, probably during the desecration of the Cromwellian period.

Considerable changes took place in the 13^th Century - the Norman Chancel was replaced by the present more spacious one, a north aisle was added, and the adjacent nave pierced with arches to give access. After the Reformation the aisle was demolished and the arches filled in.

Saint Mary's Church has one bell, alto in voice, weighing over 3cwt. It is one of the oldest in Sussex, is believed to be the last one cast by John Rufford in 1348, and is still in its original condition.