The village is located in the Blackmore Vale in north Dorset and sits above the river Stour. The present building consists of a chancel rebuilt c.1882, a 14thc N chapel; a 15thc S chapel; a nave with a late-14thc N aisle; a S aisle and a porch of c.1852, and a 15thc W tower.

The only Romanesque architectural sculpture is a pier, with capitals, that forms part of the N arcade. A 12thc font bowl also survives, now located in the S aisle.

Marnhull is not mentioned in Domesday Book.

The Salisbury Diocesan registers of 1388-1413 record a chantry to St Mary at Marnhull, and the 14thc N chapel originally appears to have had a lodging for a chantry priest attached to its W wall.

Font: It is suggested that originally this block formed the base of a mediaeval cross-shaft (RCHM: see also the very similar base with the remains of a shaft in the churchyard at Maiden Newton). Glynne visited the church in 1842, before the alterations creating the S aisle. As well as noting the Romanesque pier surviving in the N arcade, he wrote: ‘The font is in its proper place’, (presumably at the W end) ‘a plain octagonal basin on a panelled pedestal. There is in the churchyard, another font now neglected.’ The latter probably refers to the present 12thc font bowl. The other was probably the 15thc bowl that went with the surviving base. An octagonal font basin now sits loose at the W end of the S aisle, but this dates from the 19thc.