The merthyr place-name Marteconoc attests the dedication to Cynog, and his burial at the site. The presence of Cynog's torque as a relic at the site in the later twelfth century, noted by Gerald of Wales, confirms that the church was an important, and probably pre-eminent, centre of his cult.

The high valuation of the church at £30 in 1291⁠–2 is suggestive of a church of exceptional status.

An early eighteenth-century oral tale collected by Hugh Thomas says that his decapitated body came down from the neighbouring hillside, and that the church of Merthyr Cynog was raised where the head was laid to rest. The story appears to be an explanation of why two churches seem to have stood very close together (the ruins of the earlier were still evident in 1702).