Born John Horne, the son of a poultry dealer, he assumed (1782) the additional surname Tooke to honour his friend and benefactor William Tooke. He graduated from the University of Cambridge and, in 1760, became a vicar. In 1769 he helped the embattled radical John Wilkes found the Society for the Supporters of the Bill of Rights, but two years later he broke with Wilkes and created his own Constitutional Society to agitate for parliamentary reform and self-government for the American colonies.

Horne's outspoken support of the colonists led to his conviction and imprisonment (1778) for seditious libel. From 1782 to 1785 he rallied public support behind William Pitt's unsuccessful attempts to obtain reforms of Parliament. When the revolutionary events in France prompted the British government to suppress radicals, Horne Tooke was arrested in May 1794 and charged with high treason. Six months later he was acquitted by a London jury. The radical movement was dispersed by the end of the century, but Horne Tooke gained a seat in Parliament in 1801. An enthusiastic philologist, he was among the first to regard language as a product of historical development rather than as a fixed structure.