The church is large and cruciform, with a tall north tower; it stands in a triangular churchyard at the heart of the town.

The present building dates from c. 1160–70, and Norman work survives in the nave and part of a doorway, later moved to the north porch door.

The tower was rebuilt over the north transept and is a notable example of the Gothic survival style, with four stages; Pevsner states that its buttress shafts and pinnacles are "in the Somerset fashion".

The large organ case, and the altar in the south chapel, are in Arts and Crafts style from the first decade of the 20th century.

The ancient parish of Calne covered a wide area, including Berwick Bassett, Cherhill and Studley, each with churches or chapels dependent on St Mary's.