This dissertation traces the triumph of free labor in the two largest slave societies of the nineteenth-century western world: the United States and Brazil. Drawing on a range of primary sources from American and Brazilian archives, it reconstructs the intense circulation of transnational agents between these two countries from the 1840s to the 1880s. It shows how these exchanges transformed the political economies of both nations: whereas Brazil attracted American capital and expertise to modernize its economic structure and accomplish a smooth transition from slave to free labor; the United States seized the opportunity to invest, develop, and encourage free labor in Brazil, which had long been under the influence of the British Empire. As vital as chattel slavery had become to the nineteenth-century world economy, a coalition of American and Brazilian reformers proposed that an even more efficient and profitable labor system could replace it. This transnational group of free labor promoters included activists, diplomats, engineers, entrepreneurs, journalists, merchants, missionaries, planters, politicians, scientists, students, among others. Working together, they promoted labor-saving machinery, new transportation technology, scientific management, and technical education. These improvements, they reckoned, would help Brazilian and American capitalists harness the potential of native-born as well as immigrant free workers to expand production and trade. This work concludes that, by the late nineteenth century, free labor had strengthened capitalism in Brazil and the United States, making American industrialists and Brazilian planters more powerful than ever before. Consequently, in neither the United States nor Brazil did the triumph of free labor result in the advancement of social justice. In fact, from the very beginning of their campaign, free labor promoters favored major capitalists: their goal was to concentrate capital, shatter traditional ways of life, and control highly mobile workers. Free labor meant eliminating slavery while, at the same time, reinforcing proletarianization.