The parish church of Old Radnor is dedicated to St. Stephen to whom there is no other authentic Welsh dedication. It is likely that when the Normans came to Old Radnor they found a church already in existence here, dedicated to the Welsh saint St. Ystyffan, who they mistakenly understood to be St. Stephen.

The coming of the Normans to Wales saw the familiar pattern of dioceses and parishes established and the disappearance of the old kingdom bishoprics and clas churches, or minsters, which were characteristic of the Celtic church in Wales. It was a period of great change and adaption during which the Celtic foundation at Old Radnor had to move, no doubt somewhat unwillingly, with the times. 

Though in Wales, this church is in the English diocese of Hereford, the western boundary of which with the Welsh diocese of St. David's was open-ended in pre-Norman times. When the boundary was settled, which was not without dispute, Old Radnor was one of the border parishes in Powys which was transplanted ecclesiastically into the English diocese of Hereford.

The north aisle is the oldest part of the present building and was built in the late 14th century and is the only part of an earlier church to have survived the ravages of Owain Glyndwr in 1401 who is thought to have also bestowed similar attention on New Radnor and Presteign.  The aisle had to be re-roofed and the design of its windows was changed to match those of the rest of the rebuilt church. The walls of the new nave with their elegant four-bay arcades support an almost flat oak waggon roof of heavily moulded timber, which is further enriched with carved bosses where the ribs intersect. There is no chancel arch, though the south aisle has one to the former Lady Chapel and the setting of the chancel roof beams upon elaborately carved corbels and with an arched truss to distinguish it from the nave was an imaginative 19th century innovation. The window in the south wall of the chancel sanctuary is the solitary survivor of the church's original fenestration.

The perpendicular style entrance to the substantial south porch is surmounted by three niches into which sculptures were          inserted in 1882 representing our lord in the center, with our lady and St. Stephen on either side. Inside the porch the mutilated remains have survived of a large holy water stoop as has some of the original carving of the ancient door itself. Slots into which a stout wooden post could be fitted for securing the door can be seen just inside the church.

Medieval churches were places of colour, splendour, and mystery which originated in the vestments and altar coverings, the stained glass and wall-paintings, and in the holiest parts of the church being screened off. The medieval vestment chest in the north chapel and the 15th century stained glass window there, appropriately in a collegiate church, depicting St. Catherine, the patron of scholars, with the spiked wheel with which, by tradion, she was martyred, are reminders of this splendour. So too, are the medieval glazed floor tiles which were taken up and relaid in blocks in different parts of the church in the 19th century. But by far the most impressive, however, source of colour and mystery was the late 15th century screen which still separates the chancel and chapels from the nave and aisles, though the rood above it, depicting Christ upon his cross with Mary and St. John either side has gone. ( This is one of the finest screens in Wales.)

The other outstanding piece of woodwork in the church is the elegant and graceful organ case, standing on the north side of the chancel.It belongs to the 16th century and is the earliest surviving organ case in the British Isles. It was restored in 1872 when the present Walker instrument was inserted, necessitating the removal of one of the chancel parclose screens.

Both screen and organ must have been extremely expensive and one wonders how these items were paid for. It is unlikely that its casework any more than the organ itself was of local manufacture. It probably involved craftsmen from England or beyond. A modern stained glass window in the cloisters of Worcester cathedral, the Dean and chapter of which became the patrons of the parish in succession to the Mortimers and the crown, includes a picture of this instrument. This has given rise to the suggestion that it may have come originally from Worcester cathedral where it was displaced by the acquisition of a larger instrument. The existence of such an instrument at Old Radnor cannot be unconnected with the fact that John Bull, the charismatic composer and organist to the chapel Royal as well as the first professor of music at Gresham college, London before becoming organist of Antwerp cathedral, was born in the parish of Old Radnor in 1563. The parish obviously had a strong musical tradition, involving a fine instrument and talented students.