Griffith John was born on 14 December 1831 at Swansea, in south Wales. He was brought up a Christian in the Congregational tradition, and in 1840 at the age of eight was admitted to full membership of Ebenezer Congregational church, Swansea. When only fourteen he delivered his first sermon at a prayer meeting; at sixteen he became a regular preacher and was known as "boy preacher." He was subsequently trained at the Brecon Congregational Memorial College for the ministry, and then at the Bedford Academy.