Six-bay Norman arcades: plain cruciform piers and round arches with chamfered impost band and relieving arches. Aisles have C19 quadripartite vaulted roofs to each bay. To the W of the N aisle are the remains of 3 blocked clerestorey windows with quoins. Aisle windows within high blind arches. The 3 round-headed windows above the W door have steeply raked sills and narrow roll mouldings. Shallow pitched timber-panelled ceiling to nave and chancel, supported on corbel table. Flagstone floor, except around benches where it is wood planked Wooden benches with carved ends with quatrefoils. Chancel separated from nave by 2 steps and ornate iron railings with scroll-work and large fleur-de-lis finials. Reredos of pink marble with a blind arcade of round arches. Half-glazed Gothic screens with intersecting tracery divide the chancel from the chapels at the E ends of the aisles.
The Mansel chapel in the S aisle contains 4 large marble chest tombs, each bearing 2 or 3 recumbent effigies. The 3 aligned tombs to the E are contemporary, and date to the period 1611-31. They are dedicated to Sir Rice Mansel (d. 1559), Sir Edward Mansel (d. 1585) and Sir Thomas Mansel (d.1631) and their wives. The children of the deceased are depicted as free-standing weepers, sons in alabaster to the ends, and daughters in stone, mainly to the sides. Obelisks project from the top angles of the tombs. The tomb set back to the W is later and has no weepers. White alabaster and black marble, with recumbent effigies of Sir Lewis Mansel (d. 1638) and his wife. Decorated in classical style with festoons, books and shields. Marble wall monuments to other members of the family. On E wall, a memorial to Rauleigh Bussye, stepson of Thomas Mansel, and his wife: Two figures at a prayer desk between Corinthian columns supporting an entablature with heraldic emblem rising above. To the S wall, a woman praying within a round arch, to Katherine Bussye (d. 1625 aged 17).
The N chapel contains a single prominent tomb in the centre to Theodore Mansel Talbot (1839-76) who tragically died in a hunting accident. By H H Armstead and dated 1881. A recumbent and life-like effigy in a nightshirt holding a crucifix under an Early English-style gabled stone canopy. Supported on black marble columns with foliate capitals forming 3-bay arcades with moulded trefoil-headed arches. The canopy has tall gables to each bay and to the ends, with blind trefoiled arches and surmounted by pinnacles. Decorated with angels, shields and head bosses. Against the E end, an almost full-height marble wall monument, after 1711, to Thomas Mansel (d. 1705). Four marble pilasters on a plinth supporting a cornice and triangular pediment. Between the pilasters, recessed tablets record the history of the Mansel family. Heraldic emblems in centre of pediment. In front, a plain marble chest tomb dedicated to Thomas Mansel Talbot (1747-1813), and his 2 daughters, Maria Theresa and Eleanor. To the S is a wall monument to Christopher Rice Mansel Talbot (1847-1894) and his daughters, Emily Charlotte and Olivia: A rectangular tablet between pilasters decorated with festoons supporting a swan-necked pediment from which a shield rises.
The font is part of the restoration of c1810, of pink-grey marble from Caen. Bowl with dog-tooth frieze to rim, pink marble stem and tiered stone base. The polygonal pulpit is of Caen stone but dates to the restoration of 1872-3. Decorated with blind round arches and coloured marble inlay and on a red marble stem. Ornately carved mahogany choir stalls with Romanesque detail including arcades, by William Clarke of Llandaff, 1904. These were a gift of Emily Charlotte Talbot in memory of her sister, Olivia (d. 1894). Organ in 4th bay of N arcade, rebuilt by Emily Talbot in 1903.
Stained glass: W end lights designed by Burne-Jones and made by Morris & Co, 1873; Madonna and child to centre flanked by St David and St Bernard. W end of S aisle; memorial to Maria Theresa, eldest daughter of Thomas Mansel Talbot (d. 1861). East end lights have colourful stained glass of 1904-5 by Sir William Lawrence and made by Powell's. They depict Christ, the annunciation and St Michael.
Grave stone below W window of N aisle, to John Williams (d. 1811), supported on moulded stone fragments. To L of organ, a white marble tablet with swags and books to Rev. William Bruce Knight, dean of Llandaff and minister of Margam parish (d. 1845), signed P Rogers, Swansea. Two bronze plaques attached to piers of N arcade, commemorating those who died in the 1st World War, one to members of the Glamorgan Royal Horse Artillery.