Thomas was Liberal MP for Merthyr Tydfil from 1888 till 1910. He was an early supporter of the Cymru Fydd movement, but opposed its integration with the South Wales Liberal Federation, thus making an enemy of David Lloyd George.
Having made a fortune as a coal-owner, he was created Viscount Rhondda in 1915, but was excluded from political office until 1916, when he became President of the Local Government Board. As Food Controller during the latter part of World War I, he was very successful.