The Swansea Canal, on the western side of the River Tawe, was constructed between 1794 and 1798 by the Swansea Canal Navigation Company. It ran the sixteen miles from Swansea to Henneuadd, Abercraf and provided the means of transporting coal and limestone from the area to the port of Swansea. It has now been in-filled to form the Ystradgynlais by-pass (A 4067).
Arguably the town developed as a result of the heavy industries of coal and iron. These sites have now been cleared and the landscaped area redeveloped to parkland as at the Diamond Park, the site of the Diamond and Gurnos coal mines. Adjacent to the Diamond Park is the Wern Plymis Nature Reserve maintained by the Brecknock Naturalist Society.
The Brecon Forest Tramroad was built in the years 1821-1825 as the creation of a London capitalist John Christie. It was intended to open up part of the Great Forest of Brecon, where he had acquired a large estate in 1820. Eventually it was taken over by Joseph Claypon who linked it to the Swansea Canal at Gurnos. For a period of nearly fifty years the tramroad served the agricultural community and the industries of SW Breconshire, its traffic consisting mainly of coal, iron and lime. Its location can still be seen ascending the Drum Mountain from the valley floor.
Ystradgynlais is served by places of worship of all denominations. It is said that the parish church dedicated to St Cynog was established in Ystradgynlais in the 5th or 6th century- the time of the influence of the Celtic Saints in the area. Cynog was one of the sons of Brychan, King of Brycheiniog (Brecknock) and was a leader of the church in the mid-fifth century A.D. The present St Cynog's Church was opened in 1861.
The town is fortunate in having a number of open spaces for recreational purposes such as the Gorsedd Park, near the town centre, opened after hosting the National Eisteddfod of Wales in 1954. In the park is the District War Memorial commemorating the names of those who lost their lives in the Service of their Country in the two World Wars and the Coronation Park, donated to the town by the Gough family, to commemorate the Coronation of King George V in 1911.
Ystradgynlais's Recreation Ground, donated to the community by the Gough family is the home for rugby, football, bowls, cricket and tennis and is located close to the town centre
The town's heavy industries of the last century have now given way to small industrial units like the Ynyscedwyn Business Park adjacent to the Recreation Ground alongside Trawsffordd linking the A 4067, Ystradgynlais by-pass and B 4599 road. The numerous manufacturing units located here provide a valuable source of employment for the community.
The Rivers Tawe, Twrch and Giedd provide good fishing under the care of the Tawe & Tributaries Angling Association'
West of the town centre is the small hamlet of Cwmgiedd (Valley of the raging stream). The film The Silent Village, produced and directed by Humphrey Jennings in 1943, was filmed in Cwmgiedd with the local inhabitants taking the leading roles. The film was made to memorialise the people and village of Lidice north west of Prague, in Czechoslovakia. Lidice was obliterated by the Nazis in retaliation for the assassination of Reinhard Heydrich in May 1942. The village has also been used as the location of the S4C comedy TV programme 'Licyris Olsorts'
The road bridge crossing the Swansea Canal was located at what is now the cross roads at the Cwmgiedd junction of the Ystradgynlais by-pass(A 4067) which was built in the early 1970s.
Yorath is the only chapel in the village and is said to be 'the mother of all local chapels'. The present building being the third at or near this site. Members of its congregation have, over the years, contributed a great deal to the life and culture of Wales. These include Daniel Protheroe (1866-1934) who became one of the leading figures in American-Welsh music as composer, teacher and conductor. Thomas Levi (1825-1916) the author and founder of one of the best selling magazines ever published in Wales and John Thomas Rees (1857-1949) who became one of Wales' most loved conductors and composers.