Interior
The interior consists of a main nave and a south nave of equal size, separated by an arcade of six arches. The arches are of two orders, chamfered, on octagonal columns. Both have similar C19-reconstructed roofs of seven bays, using original timbers, plus a celured part of barrel shape over the chancel of each. The trusses have high collars, arched braces, and quatrefoil and trefoil apertures at top. The celures have decorated longitudinal fascias and close-set lateral plain ribs. The floor is of red quarry tiles laid diagonally, plus Goodwin's encaustic tiles in the chancel and sanctuary of the main nave. The pews throughout are in a plain Gothic style with very little decoration.
Both chancels are defined by a single step and a low oak Gothic screen, that in the south nave being a remnant of the rood screen. The pulpit and choirstalls are C19 Gothic, the panels pierced with miniature tracery, conforming to the style of the rood screen remnants. The reredos is a frieze of ogee-headed panels said to be alabaster, now painted; it has a cross at centre flanked by the Archangel and the Virgin. The altar rails are of brass on bronze decorative standards, and have a removable centre section instead of a gate. There are no stalls in the chancel of the south nave: here the east wall is panelled; the altar rails are of oak, also with a removable centre section. A brass plaque against the east wall records the formation of the war memorial chapel here. A low oak screen in the first bay of the arcade separates the two sanctuaries. Each has a C19 aumbry at right, that of the main altar being ogee-headed.
Mediaeval glass from the east window, including a fragment indicating the date 1503, is preserved in mixed condition in the second window on the south side. The east window now has a large representation of the Crucifixion with angels in the top lights and the Last Supper beneath (to John and Mary Puleston, 1872). The first window on the south side shows St James, St Mary, St John and St Winnifred, with scenes of the Nativity beneath, by Kempe (Wynne and Goodrich families, 1890). The first and second windows on the north side are to members of the Preston family of Llwyn-ynn. The first is by Westlake, 1873, showing the risen Christ; the second shows St Anne, the Virgin and Child and St Elizabeth, dated 1880. West of the porch is a window by Whall, 1893, with Christ and children, with its side lights filled with earlier flowered quarries. The other windows have textured and coloured glass.
St Mary's has a good set of wall monuments. The earliest is to Thomas ab Rice [1582], at the west, with a coat of arms, and a later monument of the same family above. Against the south wall is a monument inscribed in Latin, with an eared architrave and rounded pediment, erected by the prominent Jacobite Sir Watkin Williams Wynne to commemorate his former teacher, the Rev. Henry Price [1748], headmaster of Ruthin school, who also resisted the Hanoverian succession. To the east on the north wall is a Gothic alabaster monument to Sir John and Margaret Puleston [1908].
In the floor of the north chancel, north of the altar, is a slab to David son of Madoc: a shield charged with a leopard and roses, in front of a drawn sword [mid C14]. In the south nave there is a fine late-mediaeval parish chest.