The porch has a single cross beam on corbelled brackets. The C14 nave pointed S doorway has a continuous chamfer with hood mould and head stops, and recessed modern doors. Above the doorway is the line of an earlier porch roof.
The nave has 4-bay C14 arcades with octagonal piers, moulded capitals, and 2-centred arches with linked hood moulds, which have head stops on the S side only. The renewed cambered tie-beam roof does not follow the nave bay structure. It is 7 bays, with beams on corbelled brackets, bold painted bosses, some of which are original, and plastered panels with moulded ribs. There is no tower arch, but a pointed doorway and door with strap hinges. At the W end of the nave is an organ loft on panelled wooden posts, and with a panelled front enriched by open quatrefoils in roundels. The chancel arch is comparatively low, suggesting heightening of the nave in the late medieval period, comprising 2 orders of continuous roll moulding to a 2-centred arch with hood mould and plain stops (probably intended for carving but not carried out). Aisles have C19 trusses on corbelled brackets and boarded ceilings. In the N aisle is an ogee headed piscina, of which the former corbelled bowl has been broken off. In the S aisle the piscina also has a broken bowl, under an ogee head of which the crockets are partly missing.
The chancel has a renewed cambered tie-beam roof similar to the nave, but with curved ends forming brackets bearing painted shields, painted and carved bosses in the centre, and painted and moulded plaster panels. Above the altar the former ceilure is a 4-centred panelled ceiling with moulded and painted plaster panels.
Of furnishings, the outstanding feature are the choir stalls of c1500, probably brought here from Llanfaes Priory. The stall ends have Gothic panelling and poppy heads, and the fronts have blind Gothic panels. Seats have rounded backs and angels on the arm rests. There are 19 contemporary misericords, all with winged angels but with different heads to the supporters, and a 20th dated 1902 in similar style. The C19 octagonal font has slender clustered shafts forming the stem, and a bowl with deeply cut Gothic panels. Pews of 1902 have ends with linenfold panelling. The pulpit has similar detail. The S aisle chapel has a brass communion rail. The sanctuary has a communion rail post 1947, and diaper work to the floor. A mid C20 wood-panelled Gothic reredos has fluted shafts surmounted by angels in prayer.
There are a large number of memorials. The most significant are the medieval memorials in the porch and N aisle. The porch contains a stone coffin and lid with high-relief effigy of Joan (d 1237), wife of Llywelyn the Great and daughter of King John. It was originally at the nearby Franciscan Friary of Llanfaes, but after the Dissolution was put to mundane use until it was rescued and taken to Baron Hill in 1808, as recorded on 2 plaques, in English and Latin respectively, and finally to the church. In the N aisle is a late C15 alabaster chest tomb with full-length effigies, and the chest enriched with weepers (in the form of saints) under canopies alternating with blank shields, formerly painted. The figures are said to be William Bulkeley (d c1490) and his wide Elen Gruffydd of Penrhyn, or alternatively Rowland Bulkeley (d 1537) and his wife Alice.
In the chancel N wall is a Gothic stone tablet erected in 1809 commemorating David Hughes (d 1609), who established the town's first school in 1603 and under whose will almshouses were built in 1613 at Elusendai in Llanfaes parish. It has clustered shafts and pointed arch framing Welsh and English inscription panels surmounted by an urn. Beneath it is a brass plaque to Anne Owen (d 1694) with Coat of Arms, and to its R a small late medieval brass to Richard Bulkeley and his wife Elizabeth of c1530, in a shallow arched recess with sunk spandrels. Further R is a sarcophagus-type tablet to Capt Hugh Rowlands Williams (d 1795). A freestanding classical memorial to Charlotte Williams Bulkeley (d 1829) is by John Ternouth of London. A sculpture of a kneeling woman stands on a high base with English and Welsh inscriptions. Within the sanctuary is a high-relief memorial to Baron Bulkeley of Beaumaris (d 1822) by Richard Westmacott of London. It has a bust on a pedestal, and a mourning woman comforted by a hooded female figure bearing a cross.
In the E wall of the chancel is a memorial to Margaret Jones (d 1609) comprising a Latin inscription in an arched panel. The sides have motifs such as spade and scythe, and skull and cross bones, in relief. It is surmounted by a fluted entablature, achievement and orb finials. On the R side of the E wall is a memorial to Thomas Caesar (d 1632), comprising inscription panel, angels facing outwards to the sides, scrolled pediment with achievement and skulls R and L. Below it is a simple but large slab commemorating William Thwaytes (d 1563). In the chancel S wall is an oval inscription panel to Wynn Howard (d 1796). A marble Gothic wall tablet under a crocketed ogee arch is to Hugh Davis (d 1821) and niece (d 1869), and a marble inscription panel to Charles Yorke (d 1897) is signed (Yarwood?), the name partly worn away.
At the E end of the nave N wall, above the pulpit, is a Tudor-Gothic tablet to Rowland Williams (d 1836), by Robert Johnson of London. It has an arched panel flanked by clustered shafts, above a frieze of quatrefoils in lozenges. On the corresponding S side of the nave are brasses to William Turner (d 1904) and to the Rev Thomas Kyffin (d 1909) by Jones and Willis.
In the S aisle the N side of the chapel has a classical wall tablet to Margaret Hughes (d 1697). It has Ionic columns, apron with garland, entablature and achievement flanked by finials. On the corresponding S side is a monument to Owen Owen (d 1833) and family by Spence of Liverpool. Its panel is spanned by an arched wreath and has an apron with coat of arms.
Other monuments are simpler. In the S aisle, beginning at the E end, are tablets to the Rev Owen (d 1831), Lieutenant John Russell (killed 1918), Henry & Harriet Selwyn (d 1831 by drowning in a shipwreck off Beaumaris) by J. Harris of Bath, a brass to Charles Stanhope (d 1895), and tablets commemorating Mary Hyde Page (d 1794) and Lewis Evans (d 1711). The W wall has a tablet to Thomas Williams (d 1739). In the N aisle, beginning at the E end, are tablets commemorating John Spencer (d 1823) and wife, comprising 2 panels with pilasters and achievement, Hester Meyrick (d 1840), a double inscription panel to William Turton (d 1841) and Richard Turton (d 1835), Ann Ferrier (d 1849), a brass to Emily Greville (d 1900), comprising a cross intertwined with flowers, and tablets commemorating Samuel Hollyman (d 1728) and John Williams (d 1734).
Several windows have stained glass. The E window depicts the Crucifixion, post 1918 and possibly by J.C. Bewsey. In the chancel S wall the E end window has fragments of medieval glass, and the window adjacent to it shows the Coronation of the Virgin and Nativity, post 1918 and signed by J.C. Bewsey. The aisles have E windows by C.E. Kempe, depicting Nativity to the S and the Angel and the Shepherds to the N. In the S aisle windows are early C20 windows, of which the easternmost window depicts the presentation of Jesus at the Temple, and the westernmost window Moses and Noah, by C.E. Kempe. In the N aisle the early C20 easternmost window depicts the New Testament scenes 'suffer little children' and 'this is my beloved son', by Wailes and Strang of Newcastle-on-Tyne. The westernmost window, of similar date, depicts SS Peter and Paul, by Kempe & Tower.