The community of Trecynon began to emerge in the early years of the Nineteenth century. Then known as Heol-y-Felin (Mill Street) because it was established along the road to Llwydcoed Mill, the catalyst for the growth of the settlement was the opening of the Aberdare Ironworks at Llwydcoed in 1800. Trecynon was officially adopted during the Mid Nineteenth Century after it was the winning entry in an eisteddfod competition to name the village. Prior to 1800 there had been only one house at Heol-y-Felin, alongside Hen-Dy-Cwrdd Chapel which had been built in 1751.

Rev. John Griffith, Vicar of Aberdare, reported that;

"There is not, to my knowledge, a place in Aberdare more filthy than the neighbourhood of the Royal Oak in the same quarter of the town. 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."

Rees Hopkin Rhys reported;

"Many of the houses in this quarter are of a very inferior description, and these have no privy accommodation whatever. The new houses, here, as a rule, have one privy for two houses."

Despite the closure of the Aberdare Ironworks in 1875, the growth of the coal trade and increasing population led inevitably to the continued development of Trecynon in the later Nineteenth Century and early Twentieth Century. Edward Street was built in the 1870's, Broniestyn Terrace in 1905. St. Fagan's Church was consecrated on 31st July 1854. The church had been built to serve the needs of the growing Anglican congregation in the upper Cynon Valley .In 1848 the Aberdare British Schools (Ysgol-y-Comin) were built on an unenclosed area of Hirwaun Common and in 1896 the Aberdare Boy's County School was built on the Southern edge of Trecynon.

In August 1902 a ceremony was held to mark the laying of the foundation stone of the Trecynon Public Hall and Library by Lord Windsor and D A Thomas M.P. Funds for the construction of the Hall came from a variety of sources, including; The Carnegie Fund - £1,000, Bwllfa Colliery - £105; Lord Windsor - £50; D A Thomas - £50; whilst public subscription raised £180. The hall was officially opened on 10th March 1903 when a grand concert was organised.

Prior to its opening in 1869 the movement for a Public Park in Aberdare had been growing for several years. The proposed site of the park was to be an area of Hirwaun Common near Trecynon. In 1865 49 acres of Hirwaun common was granted to the Churchwardens and Overseers of Aberdare Parish by the Inclosure Commissioners, and in 1866 the Aberdare Board of Health were given possession of the land. Aberdare Board of Health had to apply to the Home Secretary for permission to borrow £5,000 to pay for the work, which was to be paid back in 60 half yearly installments of £161 15s 4d. The cost of this would be borne by a special levy in the rates, a levy which the inhabitants of Hirwaun were exempted from.

Many of the Bye-laws of the park reflect the desire of the Board of Health to create a haven of peace in the burgeoning industrial town. No musical instrument was to be played in the park, no games allowed and no preaching unless the Board of Health gave permission. There was to be no selling or consumption of intoxicating liquor on the park premises. Among the more surprising Bye-laws was the caveat that no person shall beat carpets in the Park and that the servants of the Local Board of health had the power to exclude from the Park any person who is "not clean in his or her person and dressed in clean and decent apparel".

In 1910 a bandstand was erected on the boating lake island, which was used in the 1930's as a focal point for a series of illuminated fetes in aid of the Aberdare General Hospital. Lord Merthyr presented the people of Aberdare with the fountain in the Park in 1911 to commemorate the coronation of King George V. The King and Queen Mary visited Aberdare in June 1912 and each planted a tree during their visit to the Public Park.

Aberdare Public Park remains as popular today as it was when first opened. The uses made of the Park inevitably changed during the Twentieth Century and the amenities offered have changed according to the demands of the Aberdare populace.
For example, after the Second World War the Park became famous throughout the United Kingdom as the home of the Aberdare Road Races. In 1956 the Park was the venue for the National Eisteddfod of Wales. It still, however, fulfills the original role envisaged by the Aberdare Board of Health, in creating a space for the exercise and recreation of the people of Aberdare.
From June 25th to June 27th 1912 King George V and Queen Mary visited Cardiff and the South Wales Valleys. On the 27th their itinerary included a visit to Pontypridd and Porth on the Royal Train, followed by a trip in a motor car through the Rhondda Fawr and on to Merthyr Tydfil. In the afternoon they traveled by motor car from Merthyr to Aberdare.

In Aberdare Lord Merthyr presented them with an address from the people of Aberdare in a ceremony at Aberdare Park. Over 8,000 children were in attendance at the Park in order to greet the Royal couple. The most famous element of the visit to Aberdare was their excursion to see a miner's cottage.

The cottage chosen was at 71 Bute Street, Aberdare and this property is still known as Queen Mary's Cottage. Following the Royal visit the residents of the house, Mr and Mrs Jones, were visited by hundreds of people from across South Wales who were anxious to see the cup from which Queen Mary had drunk. The Aberdare Leader of 6th July 1912 reported that grateful visitors had filled the cup that the King had drunk from with coppers for the Jones' baby.

As mentioned above, Hen Dy Cwrdd Unitarian Chapel had a strong connection to the reform movement of the early Nineteenth Century. The ministers Rev. Thomas Evans (Tomos Glyn Cothi) and Rev John Jones held radical views, but so also did a number of the congregation. One such member of the congregation was William Williams, more usually known by his bardic name Y Carw Coch (The Red Stag).
Born in the Vale of Neath he had moved to Aberdare by 1832 when he married a local girl. In 1837 he opened the Stag Inn, Harriet Street. In 1841 he founded Cymreigyddion y Carw Coch, a society dedicated to the Welsh literary and musical tradition. The society held its first eisteddfod at the Stag Inn in 1841, and It was under the auspices of this society that the series of 'Eisteddfodau'r Carw Coch' were held during the 1840's and 1850's. The most famous of these was that of 1853 which produced the work 'Gardd Aberdar'.
The 'Eisteddfodau'r Carw Coch' were an expression of the reformist ideals of those associated with them. In Aberdare Parish the reforming movement was expressed in cultural and religious terms and not in violence and unrest.
The Eisteddfodau'r Carw Coch sparked the growth of the eisteddfod movement in Aberdare in the second half of the Nineteenth century. In 1850 the first chapel eisteddfod was held at Siloa Chapel, and from then onwards the eisteddfod became a recognised part of nonconformist life. Inevitably, therefore, they lost the radical edge that had been such a large part of their origin in Trecynon.