This dissertation presents a new social and political history of the anti-apartheid movement, placing local South African workers at the center of global narratives of empire, U.S. imperial landscapes, and race in international relations. Based on an examination of extensive oral histories and multilingual archival materials from the United States and South Africa, Diplomacy at Work complicates the role of U.S. multinational corporations in the late apartheid era, highlighting how workers and trade unionists leveraged the private sector and reformist workplace codes of conduct. While scholars have studied the anti-apartheid movement from the angle of U.S. corporations and top-down reforms, this dissertation—drawing heavily on oral histories and other “bottom-up” sources—focuses on workers and their efforts to fight apartheid. Employed by multinational corporations, and devoted to building international networks with trade unionists and activists outside of South Africa, particularly with Black trade unionists in the United States, these workers created what I call “anti-apartheid worker internationalism.” Viewing themselves as workers first and foremost, a transnational cohort of multiracial activists used local experiences to directly influence policy and challenge apartheid through quotidian shop floor resistance. Centering the knowledge production and experiences of a multiracial coalition of workers in both the United States and South Africa, this dissertation tells a more complete story of how corporate reforms, such as the U.S.-based workplace code known as the Sullivan Principles, unfolded on the ground in South Africa. By leveraging reformist U.S. anti-racism workplace codes of conduct, and by making common cause with an increasingly robust transnational anti-apartheid movement and international labor movements, Black workers challenged U.S. multinationals and management in both the United States and South Africa. Diplomacy at Work thus places the local in conversation with the international, centering the voices of an array of everyday actors in larger narratives of U.S. foreign policy and the global anti-apartheid and anti-colonial movements.