St John's is located in St John's Crescent, a prominent Gothic Revival church occupying the centre of an oval of mid-Victorian terrace housing. The church was designed in 1854 by J.Prichard and J.P.Seddon but it was built in stages: nave 1854-5; aisles 1858-9; aisled chancel and crossing steeple 1868-70; the steeple to a revised design by W.P.James; nave west bay added in 1902 by G E Halliday, who probably also added the choir vestry. The church's appearance between 1870-1902 is recorded in a watercolour in the choir vestry. Minor alterations and additions were made by George Pace in the late 1950s. The date of the extension to the nave, with a fifth bay, is uncertain.
The walling of the church is of multi-coloured, coursed squared rubble with Bath stone dressings under Welsh slate roofs. Its plan comprises a five-bay nave with tower over crossing, chancel, north and south aisles, and vestries at north-east end. The nave is almost covered by the aisles below the clerestorey which is lit on either side by five circular cinquefoiled windows. The tower is in three stages with a stair-turret and ashlar broach spire. The roof is very steeply pitched with coped gables. The south aisle runs for six bays in front of the nave and the tower, the bays separated by buttresses.
Inside, the five-bay arcade of pointed arches are supported on circular columns. The roof features close-set double scissor braces. The lean-to aisle roofs are supported on stone arches with circular openings in the spandrels. Other features include a triple-gabled reredos; a Victorian font in the form of an octagonal limestone bowl supported on clustered red marble piers and with wooden cover; and much stained glass, the most recent dating from 1981.
Sources: extracts from Cadw listing description & J.Newman, Buildings of Wales: Glamorgan (1995), p.275

RCAHMW, 3 December 2014