Leonard Horner was born on January 17, 1785, in Edinburgh, Scotland.

He was a Scottish merchant, geologist, and educational reformer. In 1799, he entered the University of Edinburgh where he studied chemistry and mineralogy, but he left it to become a partner in his family's linen business in 1803. In 1808, he joined the newly formed Geological Society of London and was elected one of the secretaries in 1810. Devoted to the welfare of the Society throughout his life, he was elected president in 1846 and again in 1860. In 1821, he founded the Edinburgh School of Arts and was one of the founders of the Edinburgh Academy in 1823. In 1827, he was invited to London to become warden of London University (now University College London). In 1831, he moved to Bonn for two years and pursued the study of minerals and rocks, publishing numerous articles in geological journals. He briefly returned to Edinburgh to take charge of his ailing father's company (1828-1833). A radical educational reformer, he was involved in the establishment of a University College School. As a commissioner on the Royal Commission on the Employment of Children in Factories, Horner arguably did more to improve the working conditions of women and children in North England than any other person in the 19th century.

In 1807, he married Ann Susanna Lloyd (1786–1862). He died on March 5, 1864, in London, England.