In 1802 Thornton was one of the founders of the "Christian Observer", the Clapham Sect's journal edited by Zachary Macaulay, to which he contributed many articles. He was also involved in supporting the spread of Christian missionary work, including the founding of the Society for Missions to Africa and the East (later the Church Missionary Society) in 1799, and the British and Foreign Bible Society (now the Bible Society) in 1804, of which he became the first treasurer. A friend of Hannah More, he assisted in the writing and publication of her Cheap Repository tracts. In 1806, Thornton served as Manager of the newly formed London Institution.