The chancel arch is in the form of two arches, one inside the other, designed to accommodate the thick east wall, which supports the belfry. The pointed arches of the south arcade are supported by round piers with varying capitals. In the angle of the chancel and south aisle there is an early thirteenth-century chapel. Two round-headed Norman windows remain; the west window of the south aisle and the centre window in the north window of the chancel. The chancel east window has the only stained glass in the church. The "fantastically long" hagioscope, about in length and resembling a tunnel, runs from the south aisle into the chancel through one of the pillars of the arcade and the chancel arch. These architectural features allowed worshippers in the aisle to see the bread and wine being consecrated during the Celebration of the Eucharist. The church is roofed with Horsham slabs, a common material in churches in the area. The oak shingled spire of 1602 rests partly on the masonry of the old belfry, or bell turret, and partly on the east wall of the nave. A survey of Sussex church bells published in 1864 recorded four in place at St Mary's Church. Each had a diameter of and weighed . The first bell bore the inscription and an armorial shield. The second had the words above the initials and date 1602. The third bore the names of two 17th-century churchwardens, John Brooker and Edward Jupp, in their old spellings: The fourth was inscribed 1626. Another countywide survey undertaken in the 1960s stated that the second bell (of 1602) had been recast by the Mears & Stainbank firm, who had also supplied a new bell inscribed and dated 1950. This is confirmed by a brass plate dated the same year, which shows that the restoration and installation of the new bell commemorated the late John Junius Morgan, formerly of Spod Shitterton Nyetimber. The wall paintings in St Mary's were uncovered in 1882. They were treated by Professor Tristram in 1931 and later preserved by Mrs Eve Baker. Dating from the 12th to 14th centuries they consist of two 13th-century series with biblical themes together with earlier and later images. Nairn comments that "the ensemble can still give a ghostly echo of the original effect". The oldest significant painting is a medallion at the east end of the south aisle.