St Mary's occupies what may be the oldest church site in Sandwich, where a 7th century convent was founded of which nothing now survives.

There remains substantial parts of a large Norman church, despite the town having been sacked by the French in the 13th and 15th centuries, an earthquake in the 16th and the collapse of the tower in the 17th.

The north arcade was then replaced with massive chestnut columns and the immense barn-like breadth of the nave and south aisle was roofed in one span.

The tower over the south porch has a wooden belfry built in 1718.  There are monuments from several periods and some interesting fittings.