In 1898 the Diocese of Menevia was established, and the gothic style Roman Catholic parish church of St Mary became the procathedral, later the cathedral.
The church dates back to Medieval times, the tower and the north and south doorways are of this period as are the remains of three sepulchral monuments.
The large wall painting depicts four of the Acts of Mercy from St Matthew 25:35-40, feeding the hungry, giving drink to the thirsty, clothing the naked, and receiving strangers.
In the north chapel is one of the finest monumental effigies in Wales, that of John ap Elis Eyton who died in 1526 and his wife Elizabeth who died in 1524. John fought on the winning side in the Battle of Bosworth in 1485 and was rewarded with large estates in Ruabon which were later owned by the Wynnstay family. The involvement of the Wynnstay family dates back to 1719, and the earliest monument to members of the family can be found in the North chapel and depicts Henry Wynn dressed as a preacher with Sir John Wynn and his wife kneeling beside him. In the south chapel there is a large monument to Sir Williams Watkin Wynn (3rd Bart) by Michael Rysbrack. The fourth baronet married Lady Henrietta Somerset in April 1769 and she died in July. A monument to her memory was made by Joseph Nollekens and can be found at the west end.
The font was commissioned from Robert Adam for the baptism of Sir Watkin's son in 1772. The church has a variety of stained glass. The east window is to the memory of Marie Nesta Williams who died in 1883 and shows the crucifixion, Transfiguration and the raising of Jairus' daughter. Other stained glass windows in the church show a variety of biblical scenes including parable of the vineyard and the raising of Lazarus. The lych gate is the parish church memorial to the fallen of two world wars and is built of Cefn stone and Wynnstay oak.