The 12th- and 13th-century church, restored in the Victorian era, serves a large rural area of the Sussex Weald, covering three villages[1] (each with current or former chapels of ease of their own) as well as the ancient settlement of Slaugham.

By the time of the Norman Conquest, the historic county of Sussex was divided into six areas called rapes, each based on a town with a castle.

[6] The oldest parts date from the early 12th century and consist of the north wall, a single doorway and other fragmentary remains in the nave, and the font.

[1][4][7] A chancel would have been built at that time as well to make a simple two-cell church, but no 12th-century fabric survives: it was rebuilt in the early 14th century.

[4][6] Its main feature is a memorial of "exceptional quality",[7] considered one of the best in Sussex:[1][12] it depicts Richard Covert (who died in 1579) kneeling with seventeen other figures representing his wives, sons and daughters.

[8][10] He added the top section of the tower, widened the south aisle further and removed most of the wall between it and the Covert Chapel (creating an archway instead), added a third bay between the chancel and the Covert Chapel, and built a vestry and organ chamber on the north side.

[7] Alterations continued later in the 19th century: a second vestry was built on the north side in 1879, the organ was moved again, and the tower was given a clock-face in 1881.

[1][12] Burials include Catherine Matcham, one of Lord Nelson's sisters;[15] and a table-tomb dated 1615 is one of the oldest such graves in Sussex.

[1] Eccentric and bad behaviour by clergy and parishioners was a common theme at Slaugham in earlier centuries.

[17] Ecclesiastical court records show that St Mary's Church was the scene of a violent crime in 1601, when Richard Woods was "charged with breaking Thomas Mutton's head in the churche" [sic];[17] while on 29 June 1621 another man was found beating his wife instead of attending the Sunday service.

[22] St Mary's Church does not have a homogeneous external appearance and cannot be attributed to one era: the gradual change and growth of the building over the centuries is more apparent.

[4][9][12] Sussex stone rubble is the main building material, although there is some ashlar as well (for instance in the walls of the Covert Chapel).

[27] Slaugham has always been a large parish: it covered 5,482 acres (2,218 ha) in 1940,[4] most of which was sparsely populated Wealden countryside.

The three other villages in the parish—Handcross, Pease Pottage and Warninglid—each had their own chapel of ease for Anglican worship, controlled from St Mary's Church;[4] the three buildings survive, but only one is still in religious use.

[4][28] All Saints Church, at 51°03′09″N 0°12′08″W / 51.0525°N 0.2023°W / 51.0525; -0.2023 on the road to Horsham, is a small structure of red brick with a large, distinctive bell-turret at its west end.

[29] When the A23 London–Brighton road was diverted in the late 1950s to run past the village and widened at the same time, the church had to give up some of its land.

[4][31] Architect Francis G. Troup designed a basic chapel of ease for the community in 1935, about a century after one had first been suggested.

[31][32][33][34] The chapel, which cost £1,200 (£105,000 in 2025)[24] and had a tall vaulted interior lit by lancets,[31] was sold in 2009, and Mid Sussex District Council gave planning permission for its conversion into a house.

[39] It later became a chantry for the benefit of the Poynings family, members of which held the manor and lordship of Crawley until 1429 when the last descendant died.

[40] The church was later dedicated to St John the Baptist and opened for public worship in the growing town.