Described by William Cowper as 'one who wears a coronet and prays', he was born at Marylebone on 20 June 1731 and educated at Westminster School and Trinity College, Oxford. Because of his father's early death in 1732, he succeeded to the title in 1750. Through his half-brother, Frederick, Lord North (with whom he went on the Grand Tour in 1751-1754), he found himself drawn into a public career somewhat against his natural inclinations and gifts. In 1765 he accepted the post of President of the Board of Trade and was made a Privy Councillor. As Colonial Secretary from 1772 to 1776, his firm belief in parliamentary jurisdiction over the colonies was tempered by a natural disposition to seek conciliation. But events, including the 'Boston Tea Party' in 1774 and the War of Independence that followed, led to his resignation, though he remained in the Cabinet as Lord Privy Seal (1775-1782). As a person he was highly regarded by George III. Dartmouth College in New England (incorporated 1769) was named in his honour. He died at Blackheath, London on 15 July 1801.
Both Dartmouth and his wife were committed evangelicals. He embraced Methodism through the preaching of George Whitefield and he was closely associated with William Romaine and Lady Huntingdon. Charles Wesley described him as 'that most amiable of men'. John Wesley also knew him and sought his support in an attempt to unite the evangelical clergy in 1764. On the eve of the Revolutionary War Wesley wrote to him and to the Prime Minister Lord North, urging against the use of force against the colonists on both moral and prudential grounds. One of his homes, Sandwell Park, West Bromwich, was close to Francis Asbury's boyhood home; both attended All Saints Church and the Methodist society in Wednesbury.