Brecon Cathedral was so designated relatively recently, in 1923, following the establishment of new Diocese of Swansea and Brecon, before which this area was part of the St David's Diocese. The building however is much older - some small sections date from the late 12th century, though most of the structure is from the 13th and 14th centuries, and during all this time the church was part of a Benedictine priory, that was founded in 1093 by a follower of Bernard de Neufmarche, a Norman knight, and closed in 1538 by order of Henry VIII. The priory was situated on the site of an earlier, Celtic-era church.
Following the Dissolution of the Monasteries, the church was given to the parish of Brecon, known initially as Church of the Holy Cross and latterly as the Priory Church of Saint John, while the associated monastic buildings were either demolished or used for other purposes, as is still the case today, with a heritage centre, the diocese administration centre and clerical housing all occupying parts of the former priory. Some reconstructions and repairs to the cathedral have been necessary in recent times but the core is little changed from its medieval peak, and represents a fine example of the Early English (the chancel) and Decorated Gothic (the nave) styles. While small and relatively plain, compared to some of the great English cathedrals, the building shares the characteristic features of grand architecture, stained glass windows, side chapels, and ancient monuments.
Brecon Cathedral is served by a pay-and-display car park on the north side, off Priory Hill (B4520) a little way north of the city center; from here a path leads to the main entrance, through a porch into the nave. The parking area adjoins the graveyard, which is shaded by large trees. A fence prevents walking all the way around the cathedral, though the south side can be viewed via a driveway that leads to the various subsidiary buildings, and to the secluded, walled lawn that borders the south frontage. The exterior has a rather austere appearance, with study, functional architecture, lacking the ornate carvings, pinnacles and tracery of many similar churches. All is constructed out of the local Old Grey Sandstone, grey or dull red in colour. The cathedral is 205 feet long, centred on a square tower, above the choir, or crossing, which has north and south transepts at either side, and sits between the nave to the west and the chancel to the east.
The nave occupies about half the length of the building and is lined by aisles on both sides, separated by three pairs of octagonal pillars. The east end of the north aisle is occupied by St Keyne's chapel, divided from the nave by an fine carved wooden screen, probably from the 14th century. The chapel was originally dedicated to the cordwainers (shoemakers), according to the tradition of local tradesman having their own section of the church in which to pray; a corresponding chapel in the south aisle, no longer present, was dedicated to the weavers. The upper portion of the nave is illuminated by arched clerestory windows, while above these is a wooden ceiling, installed in 1875 as part of restorations carried out by Sir Gilbert Scott. The west front has a fine stained glass window, installed in 1898. The walls of the nave, and the two transepts, are lined with numerous monuments to local people, mostly from the 18th and 19th centuries. The west end contains the baptismal font, featuring carvings of geometric designs and mythical figures, and dating at least from the 12th century if not earlier.
Dating from the late 12th or early 13th century, the chancel is the oldest surviving section of the cathedral - a finely proportioned, rectangular enclosure lit by three pairs of slender, three-light windows, with a five-light window at the east end, above the high altar and the reredos, this latter an elaborate stone installation designed in 1937. The fan-vaulted stone ceiling is relatively recent, built in 1862, though it follows the intended design from the 13th century, which was not originally implemented due to lack of funds, and instead a simpler wooden construction was used. The north wall of the chancel contains a doorway to the Harvard chapel, also entered from the east side of the north transept; this has whitewashed walls, a dark wooden roof and a recent stained glass window at the far side. The chapel was built in the 14th century to honour the local Harvard family, and is now dedicated to the South Wales Borderers, the 24th Regiment. The south side of the chancel adjoins the sacristy, and also a short passageway leading to St Lawrence chapel, another recent addition (1929). Beside the entrance is a bronze effigy of Edward Bevan, the first bishop of the diocese. Other notable monuments include a wooden female figure from the mid-16th century, the only surviving part of a larger memorial-tomb to the Games family of Aberbran, the remainder of which is presumed to have been destroyed during the Civil War.