John Thomas, later celebrated across Europe by his bardic title Pencerdd Gwalia ('Chief Musician of Wales'), was born on 1 March 1826 in Bridgend, Glamorgan, the son of a tailor who also played the clarinet in the town band. Music ran deep in the family: John was the eldest of seven children, four of whom became harpists in their own right, most notably his brother Thomas Thomas. At just twelve years old, John won a triple harp at the 1838 Abergavenny Eisteddfod, a victory that changed the course of his life.
His talent caught the attention of Ada, Countess of Lovelace, daughter of Lord Byron, who sponsored his studies at the Royal Academy of Music from 1840 to 1846. He remained there as a Fellow and went on to tour concert halls across France, Germany, Russia, Austria and Italy through the 1850s. At the 1861 National Eisteddfod in Aberdare he was formally invested with the title Pencerdd Gwalia, cementing his status as Wales's foremost harpist of the Victorian age.
In 1871 Thomas was appointed harpist-in-ordinary to Queen Victoria, a post he held until her death in 1901, after which he continued serving Edward VII. That same year he founded the London Welsh Choral Union. He taught harp at the Royal Academy of Music, the Royal College of Music and the Guildhall School of Music, and published over two hundred works, including four volumes of Welsh Melodies that did much to preserve and popularise Welsh airs for future generations. He died in London on 19 March 1913 and was buried at West Hampstead Cemetery.