The church is entered by the west porch in the base of the tower. The porch is stone-flagged and there is a stone staircase (to the gallery and tower) at left. The tower has bells and a clock of 1848.
Pointed door to nave, entering beneath the timber support structure of the gallery. The exceptional quality of this church is evident in the first view of the nave and chancel. The nave roof is of four bays with arch-braced collar beam trusses with cusped struts and windbraces. The nave is a little wider than the north aisle (which was the original nave), from which it is separated by an arcade of four pointed arches on octagonal piers. The nave and aisle are stone flagged and planned as one for the purpose of setting out the pews, which are separated by two main passages, a passage at rear and a passage to the south door. Panelled pews with carved top rail and ends. C18 pulpit to right, with sound-board. At the chancel steps, centrally, is a fine brass chandelier, known as the Seren Silin, made by Richard Roberts of Birmingham, a native of Llansilin, in 1824; it has a lowering mechanism.
The west gallery is supported by a late mediaeval timber frame (large timbers, cusped braces - possibly reused from a rood screen?) Above the west gallery is a Commandments board with Moses and Aaron, painted on canvas, which was formerly in the chancel of the north nave; it carries the names of late C18 churchwardens. The gallery balustrade has C18 splat balusters. Beneath the gallery is a poor-box dated 1661. Above the gallery hangs a timber chandelier of the C17. At east end of the north aisle are two preserved box pews, and a low screen to the vestry formed of reused box-pew wainscot. C17 font and cover at the west end of the nave.
Small step up to chancel. The chancel is paved in red quarry tiles with strips of encaustic tiles. Its high point is the three-bays of C15 celure of barrel form with traceried panels (originally there was a similar celure to both chancels). It is tied by three large chamfered and stopped beams with cusped struts above. Double choirstalls each side by Baker, incorporating reused carved C17 wainscot. Organ at left, complete with original hand-blowing lever and indicator. Vestry to north, at the nave floor level.
Two steps up to sanctuary, with a hardwood Communion rail without gate. Carolean altar table, one end uncarved. Panelling on full width of the east wall with returns each side.
Monuments include a Baroque carved memorial on the south wall to Sir William Williams of Glascoed, Speaker of the Commons, with urns, arms, cherubs and pilasters, 1700. Another large Baroque memorial on the north wall is to David Morris of Penybont, with an inscription detailing the history of his property, 1719, a splendid example of the use made of church memorials to publicise title. This memorial has a fine wrought iron grille at the foot and a plaster coving above. Display of former box-pew brass nameplates on wall of north aisle. Also on the north aisle wall are the Royal Arms, in plaster, now painted white, but paradoxically those of Queen Anne post-1707 together with a Foulkes monument of 1762, in time of George III, evidently put up at the same time. Was this an anti-Hanoverian gesture?
The east window features the four Evangelists and the Ascension, by Powell and Sons. The east window of the north aisle (installed 1875) commemorates Huw Morris of Eos Ceiriog, Royalist satirical poet, d.1705 (buried in churchyard).