Why have the South American governments defied global trends towards immigration restriction and passed laws that expand immigrants’ access to entry, legal residency, and rights? To answer this question, this dissertation examines the case of the Mercosur Residency Agreements (2002), which are the first legislation to make legal status a substantive right. Even though Mercosur is an intergovernmental organization with non-binding policies, by 2009, Argentina, Brazil, Bolivia, Chile, Paraguay, and Uruguay had adopted the Residency Agreements as national policy. This study identifies the factors, actors, and mechanisms that shaped the wide adoption of these agreements. To achieve this objective, I draw on 130 in-depth interviews, data from organizational archives, and original compilations of immigration policies as well as trade, economic, and migratory flow indicators from 1970 to 2017. Chapter 2 uncovers the power dynamics that shaped the ratification of the Residency Agreements. I argue that state diplomats engage in political mobilization at multiple levels of decision-making within and across states to address resistance against the Residency Agreements and secure their ratification and diffusion within South America. Chapter 3 examines why governments in Argentina, Brazil, Bolivia, and Uruguay have completely internalized the Residency Agreements while those in Chile and Paraguay have not. In the four countries that fully internalized the agreements, the governments were committed to regional integration. These four countries also had established coalitions of state and non-state actors that supported immigration reform proposals that expanded immigrants’ rights. These coalitions successfully tied the bills to broader legal-bureaucratic reforms that sought to improve democracy. Although all six states have internalized the Residency Agreements to some degree, Chapter 4 assesses why many qualifying indigenous immigrants from Bolivia remain undocumented. I find that indigenous Bolivians migrate through unofficial channels and remain irregular because they either resist state legal logics, the Bolivian state does not provide many of them with the identification documents, and receiving states implement their immigration laws in a manner that imposes excessive hurdles to legal residency. Overall, this study expands our understanding of immigration governance in the Global South and identifies limitations to laws that omit indigenous practices.