The London-based NGO Anti-Slavery was founded in 1839. For almost all its history, it remained a small group, working primarily through informal links to British parliamentarians. Pressure from the group made a significant though indirect impact on the 1926 League of Nations Slavery Convention and the 1956 United Nations Supplementary Convention. Anti-Slavery’s focus has shifted from chattel slavery to contemporary forms of slavery, which remain poorly defined in international law. This article examines both the evolution of Anti-Slavery and the League’s and United Nations failure to establish an effective monitoring group, which Anti-Slavery has consistently pressed for, albeit unsuccessfully.