It dates from the early nineteenth century and developed as a result of the opening of the Aberdare Ironworks at Llwydcoed in 1800.

[2] With the opening of the ironworks at Llwydcoed in 1800, the settlement became an urbanised village, with housing and amenities (such as the 1855 Heolyfelin Chapel) built away from the Heol-y-Felin itself.

[3] The first buildings in Trecynon were Llwydcoed Mill, the Hen-Dy-Cwrdd in 1751 and a single solitary house built next to it.

Mill-Street proper is in a very bad state from the ash-heaps of rubbish and filth thrown into and lying on the centre of the road.Likewise, local industrialist Rees Hopkin Rhys reported: Many of the houses in this quarter are of a very inferior description, and these have no privy accommodation whatever.

[1]Rammell's report led directly to the formation of the Aberdare Local Board of Health the following year.

Population continued to grow steadily in the late nineteenth and early twentieth century, as a result of the steam coal trade.

In August 1902, the foundation stone of the Trecynon Public Hall and Library was laid by Lord Windsor and D. A. Thomas, M.P.

The Aberdare College campus site of Coleg Morgannwg is situated within the northern part of the village, on Cwmdare Road.

Unitarians cared less for their buildings than other nonconformist denominations who built increasingly grandiose edifices, such as Calfaria, Aberdare.