This dissertation examines U.S. views of Mexico from the end of the U.S.-Mexico War in 1848, to the end of the first phase of the Mexican Revolution in May 1911. During this period numerous Americans saw Mexico as a laboratory to test their ability to transform a country seemingly in need of guidance. Americans, however, struggled to define the role of the United States: whether it was solely to be a model for other nations to follow, or whether Americans should be actively involved in this process. In the years after the U.S. Civil War, a diverse group of Americans, especially missionaries, investors, and working-class activists, saw Mexico as a nation in need of change and sought to affect its transformation through the means of informal imperialism. Yet they vigorously disagreed whether this transformation should occur in religious, political, economic or social terms. Despite these differences, they all believed that Mexico could be reshaped in the image of the United States. Their views thus provided a powerful counter-narrative to persistent U.S. images of the Mexican people as irredeemable because of allegedly inherent inferiorities based on race, religion or culture. The dissertation also examines the role of Mexican actors in attracting, resisting and altering U.S. informal imperialism. These Mexican actors included government officials who petitioned for U.S. assistance during the French Intervention (1862-67) and the Porfiriato (1876-1911); dissident Catholic priests who requested aid for the fledgling Protestant movement in Mexico; and Mexican liberal exiles from the repressive Díaz regime, who sought U.S. support in bringing a democratic government to Mexico. More generally this dissertation challenges scholarly assessments of the United States as an isolationist nation during the mid-to-late nineteenth century, before the embrace of formal empire after the War of 1898. Though different groups of Americans would come to divergent conclusions about the foreign policy of the United States, a close analysis of U.S. efforts to reshape Mexico reveals an outward-looking and internationalist public that took seriously its self-image as a nation destined to transform the world.