Ironworks were established at Llwydcoed and Abernant in 1799 and 1800 respectively, followed by others at Gadlys and Aberaman in 1827 and 1847. These have not been worked since about 1875. Prior to 1836, most of the coal worked in the parish was consumed locally, chiefly in the ironworks, but in that year the working of steam coal for export was begun, pits were sunk in rapid succession, and the coal trade, which after 1875 was the chief support of the town, soon reached huge dimensions. There were also several brickworks and breweries. During the latter half Of the 19th century, considerable public improvements were made to the town, which became, despite its neighbouring collieries, a pleasant place to live. Its institutions included a post-graduate theological college (opened in connexion with the Church of England in 1892, until 1907, when it was removed to Llandaff).
Aberdare, with the ecclesiastical parishes of St Fagan's (Trecynon) and Aberaman carved out of the ancient parish, had twelve Anglican churches, one Roman Catholic church (built in 1866 in Monk Street near the site of a cell attached to Penrhys Abbey) and at one time had over fifty Nonconformist chapels. The services in the majority of the chapels were in Welsh. The urban district includes what were once the separate villages of Aberaman, Abernant, Cwmbach, Cwmaman, Cwmdare, Llwydcoed and Trecynon. There are several cairns and the remains of a circular British encampment on the mountain between Aberdare and Merthyr. Hirwaun moor, 4 miles to the N.W. of Aberdare, was according to tradition the scene of a battle at which Rhys ap Tewdwr, prince of Dyfed, was defeated by the ailied forces of the Norman Robert Fitzhamon and Iestyn ab Gwrgan, the last prince of Glamorgan.
Aberdare was the birthplace of the Second World War poet, Alun Lewis, and a plaque commemorating him is to be found, including a quotation from his poem, The Mountain over Aberdare.