It included most of the valleys of the Rhondda Fawr and Rhondda Fach rivers. Initially a sparsely populated wild area of upland sheep-walks, it became in the mid 19th century a dynamic centre of the coal mining industry, with a large, new industrial population.
The parish was large, with an area of 10127 Hectares, but had only 542 inhabitants in 1801. It stretched from the confluence of the Rhondda rivers at Porth, over the mountain as far as the Vale of Neath. It was divided into four townships or hamlet (place)s: Home (between the rivers), Clydach (south of the Rhondda Fawr), Middle (the upper part of the valley) and Rhigos (north of the mountains).
During the 19th century, the population of the parish increased as follows:
Lower-quality coal from the Upper Coal series was worked in a small way in Trealaw as early as 1807, but the development of the Rhondda steam coal gave rise to the rapid population growth. The development began with the start-up of the Bute Merthyr colliery in Treherbert in 1855. In the Rhondda Fach, the first coal was mined in 1862 at Ferndale. The Taff Vale Railway reached Treherbert in 1856. Collieries then rapidly developed along the valley, with the lower part of the valley developing last because of the deeper pits required to find the steam coal in that area. By the end of the century, mining villages formed an almost continuous urban strip along both valley floors, with coal mining and its ancillary trades virtually the sole industry.
In 1877, the Hamlet of Rhigos was made a separate civil parish, and the remainder of Ystradyfodwg, together with the adjoining Rhondda valley portions of the parishes of Llanwonno and Llantrisant, became the urban sanitary district of Ystradyfodwg. This extended district became Ystradyfodwg Civil Parish and Urban District in 1894, and was renamed Rhondda Civil Parish and Urban District in 1897. See Rhondda (district).