The church tower dates from the fourteenth century and is a Cheshire-type construction. It has diagonal buttresses, a west door with Decorated wave moulding, battlements but no pinnacles. There are two light bell-openings and a stair tower in the north-east angle. The south wall of the church shows places of medieval work with a blocked thirteenth-century doorway to the east of the porch. Inside, a pre-Perpendicular roof-line is seen above the tower arch. There is evidence of a Perpendicular remodelling ( c.1350-1530) in a few remaining features: a north window, the aisle west windows, a pair of crocketed buttresses at the east end, and cusped niches internally on each side of the east window.

The north and south doorways are of this period. There are three badly mutilated sepulchral monuments dating from the first twenty years of the fourteenth century. Colin Gresham identifies two as mutilated effigies. The first is a military effigy with an inscription round the edge of the shield' Here lies Iorwerth ab Awr', an ancestor of the Lloyd's of Plas Madoc. The heraldry on the shield is a Lion rampant guardant. The second has the same heraldry on the shield and reads 'Here lies Hywe1...' the rest is lost. Gresham notes that the combination of hands conjoined and shield placed over the body and arms is unique amongst the effigies of North Wales. The third is an heraldic slab which again fails to reveal much about the person commemorated and is a memorial to one Llywellyn ap.

It is possible to reconstruct the appearance of the church before the radical reordering begins in the middle of the eighteenth century. The rural dean in 1749 described the church as consisting 'of three small Isles' .This accords with the drawing of Thomas Dineley, Chaplain to the Duke of Beaufort, in The Account of His Official Progress Through Wales in 1684. The earliest description of the church is by that extraordinary poet Thomas Churchyard whose topographical verses, The Worthiness of Wales (1587), extols:
'Ruabon Church is a fayre peece of worke... with pillars large and wide... The trimmest glasse, that may in window bee (wherein the roote, of Jesse well is wrought) ...Yea all the glasse of churche was deerely bought'.

In 1754 the itinerant Irish bishop, Dr Richard Pococke, noted 'There are remains of some painted glass on the east window, and of a fine carved Rood loft'. From these vague descriptions we have some idea of the nature of the church from its enlargement before the Reformation to its alteration in 1770 and 1870. It was a reasonable size with three aisles, a plain tower, a Perpendicular arcade and a simple exterior. A fine carved rood screen and loft divided the nave and chancel. The roof richly carved, the walls decorated with paintings and the windows adorned with fine medieval stained glass. Churchyard noted the effigy of John ap Elis Eyton and his wife Elizabeth. 'A monument...amid the queere. I spyde' but he had no knowledge of the medieval wall paintings which were hidden in obedience to the Order in Council of 1547 directing the 'obliteration and destruction of all popish and superstitious books and images so that the memory of them shall not remain in their churches and houses' .