The original St Mary's Church dates from the sixteenth century, and probably incorporated an earlier chapel. There were several phases of building. The porch was probably of 15th or 16th century origin, the chancel probably early English or possibly early Decorated (so late 12th to mid 14th century). The nave is thought to have been later, with the final addition of a Bath stone belfry. Interior alterations and additions, including changes to the chancel arch, were carried out in 1810.
The entrance porch was on the south side with two steps down into the nave. The font was on the left of the entrance. From the nave there was a step up into the chancel where the altar was at the east end. Under the chancel were two brick vaults. A late addition was a brick cupboard on the north east corner of the chancel. More information is provided in the report by Peter Webster.
The churchyard dates from 1616, when the church obtained a license for baptisms, marriages and burials. Prior to this the interior of the church was used for burials, excavations carried out in 1973 having revealed fragments of hundreds of skeletons.
After a new St Mary's Church was built in 1883-4 at the junction of Church Road and Penlline Road, the old church was demolished in 1904. In 1903 it had been described as in a ruinous state after two decades of neglect. In the course of the demolition some roof timbers were found to be inscribed with the names of Llandaff carpenters with 17th century dates.
In 1889 David Jones of Wallington made the following notes about Whitchurch, the Old Church:-
The church consists of nave and chancel - nearly continuous - rudimentary arch only between the two - modernised & ceiled in early part of this century. There is a "beautified" bell turret of that period. South porch pointed doorways the outer one old and moulded - inner one of later date - bevelled on one edge.
A very full churchyard. A great many more inscriptions of early part of century, chiefly (where good) of Melin Griffith people
David Jones also recorded some of the memorials then present in the church and churchyard.
A description of the church appeared in the South Wales Daily News in 1899, some five years before the building was demolished:  The church certainly makes a pretence of standing, but there is not a pane of glass left in either nave or chancel. The slates are falling from the roof, the roof timbers are trying their best to hold together, but they are fast rotting, and the walls which support them are crumbling away. The dozen or so of broken chairs are piled in the nave. The gallery is dangerous to tread upon, but, strange anomaly, the altar, with its altar cloth decayed and saturated by every shower of rain, still stands behind the Communion rails, and on the pulpit desk still rests the damp and torn Prayer Book, inscribed "Whitchurch Parish Church." The church, for sketches of which we are indebted to Mr George E. Halliday F. R. I. B. A., architect of Cardiff, is not without interest, more on account of its monuments than from any architectural feature. Perhaps the chief features architecturally are the jambs of the porch entrance, which seemed to date from the 15th century. They were probably brought from an earlier building. The earliest memorial slabs date from 1645 to 1697, but the majority are of the 18th century. In the churchyard are the names of a great many well-known and wealthy families of the neighbourhood, and it seems a pity that something is not done to preserve the church and its surroundings, which would not be a difficult task.
The churchyard continued to exist up to 1973 when the Cardiff Council Parks Department began work to convert it to a public garden. As part of this work the surviving gravestones were plotted on a plan and the inscriptions were recorded where possible, and these documents have been deposited with Glamorgan Archives. Some these transcriptions also appear in a booklet prepared by the Glamorgan Family History Society.
Sources of Information
In general, the information in this section is taken from a report by Peter Webster of the Excavation of old St. Mary's Church Whitchurch, 1973, carried out by students of the Extra-Mural Department of Cardiff University and directed by Graham Morgan. Copy held by Glamorgan Archives reference: DXKB/14.
Other sources are: