In 1871 he was one of the 450 voices of the South Wales Choral Union, better known as Cor Mawr Caradog, under the baton of Griffith Rhys Jones or Caradog.

William Thomas came to Treorchy, accompanied by his parents and wife, to become chorusmaster of Noddfa Chapel at the end of 1873. He was a deacon of the chapel for thirty years. A leading member of the local Cooperative movement, he was also appointed the Attendance Officer for Ystradyfodwg School.

A well known musician and one of the most important conductors in the country,  William Thomas led hundreds of gymanfaoedd ganu (or singing festivals) throughout the UK.

In 1885 he took up the baton at the newly formed Treorky Male Choir and during the next ten years led them to international acclaim with two first prizes at the Royal National Eisteddfod of Wales. In 1895 he conducted the Choir at its Royal Command Performance for Her Majesty Queen Victoria and the Royal Family at Windsor Castle.

With the growing demand for international concert tours, William Thomas formed the Treorchy Royal Welsh Male Choir in 1896 from members of his original Treorky Choir and the new entity enjoyed a celebrated career for almost a century.

During his time as Conductor, William Thomas led the Choir on a year-long tour of the USA in 1906 where they sang before President Theodore Roosevelt. The Choir went on to enjoy further tours of the USA in 1907, South Africa, Australia and New Zealand. In 1916 William Thomas retired due to ill health and moved to Cardiff, where he died in 1926.