Thornton was one of the founders of the Clapham Sect of evangelical reformers and a foremost campaigner for the abolition of the slave trade. A close friend and cousin of William Wilberforce, he is credited with being the financial brain behind their many campaigns for social reform and philanthropic causes which the group supported. For some years Thornton and Wilberforce shared a house called Battersea Rise which Thornton had bought in 1792. The cousins spent much time here co-coordinating their activities and entertaining their friends. After their marriages in 1796–97 they continued to live and work in close proximity for another decade.