Most of the slaves, who were thus unconditionally freed, returned without any solicitation to their former masters, to serve them, at stated wages; as free men. The work, which they now did, was found to better done than before.

The Clarkson School Wisbech is named after him as is Thomas Clarkson Academy. The "box" of his collection became an important part his public meetings and was an early example of a visual aid. For Rees's Cyclopaedia he contributed the article on the Slave Trade Vol 33 1816.

In 1840 he was the key speaker at the Anti-Slavery Society (today known as Anti-Slavery International) conference in London which campaigned to end slavery in other countries. In his later years Clarkson campaigned for the abolition of slavery worldwide. He helped found The Society for Effecting the Abolition of the Slave Trade and helped achieve passage of the Slave Trade Act of 1807 which ended British trade in slaves.