Founder of Welsh Calvinistic Methodism, born at Trevecka, Talgarth on 23 January 1714. Initially intended for holy orders, his father's death in 1731 interrupted his education and he began his career as a schoolmaster. Converted in 1735, he was soon involved in an itinerant ministry and in setting up societies. He made several applications for ordination, but was turned down, and therefore remained an Anglican layman throughout his life.
He met George Whitefield and the Wesleys in 1739; a long period of co-operation followed despite doctrinal differences with Wesley. He often travelled to London to assist at Whitefield's Tabernacle, but during the latter half of the 1740s he was suspected of heresy. This, coupled with his autocratic manner and the presence of 'Madam' Sidney Griffith as his travelling companion, led to tension which resulted in his separation from Daniel Rowland and the majority of Welsh Methodists in June 1750. Following the death of 'Madam' Griffith in 1752, he retired to Trevecka and established a Christian community that became known as the 'Trevecka Family'. With his elder brother Joseph he was involved in the formation of the Brecknock Society (the first of its kind in Britain) for ‘the encouragement of Agriculture and Manufactures’ and was one of its most prominent and influential members. In the late 1750s, he served as Captain Lieutenant in the Brecknockshire militia, formed in response to the threat of a Catholic invasion during the Seven Years' War. He equipped five young men from his community to join the 58th Regiment at Hereford. When the regiment was disbanded in 1762, he was reconciled with his former co-revivalists and resumed itinerating. By then, a new revival had broken out in Wales which centred on Llangeitho, and Harris was marginalised by a new generation of Methodists. Following the death of his wife in 1770, Harris' own health began to deteriorate. He died on 21 July 1773 and is buried at Talgarth.