Following the 2011 endorsement of the UN Guiding Principles on Business and Human Rights (UNGPs), states have begun to implement National Action Plans (NAPs) to operationalize the UNGPs. Using a case study approach and applying a conceptual framework for polycentric governance, this article aims to provide an early assessment of the effectiveness of NAPs adopted by the United Kingdom and the United States to combat one of the worst human rights abuses in global supply chains: modern slavery. This study demonstrates that both NAPs contain elements addressing the governance gaps surrounding modern slavery, such as enacting new laws, adapting existing regulations, strengthening multi-stakeholder mechanisms for business accountability, and promoting innovation. However, it is argued that the NAPs themselves were not the catalysts for the majority of these measures. This article concludes that states should optimize the five characteristics of polycentric governance outlined in this study to improve the relevance and effectiveness of NAPs as drivers of change.