This article investigates how Progressive Era writers, both popular and scientific, helped to construct multiracial identities alongside competing efforts to enshrine race into strictly black and white terms. Existing scholarship on race in the Progressive Era has not sufficiently analyzed the presence of multiracial populations. Instead, scholars have treated state and federal efforts to police racial boundaries, namely through anti-miscegenation laws and the census, as evidence that multiracial persons were a legal impossibility. However, scientific and popular writing on Appalachia provides a conceptual space in which multiracialism was not only a conceptual possibility, but was engendered. Appalachia took on increased importance during the Progressive Era as both intellectuals and reformers used the region to frame their anxieties about the limits of modernity and the threat of racial mixing. The region was home to white mountaineers who appeared arrested in time, existing in uncomfortable proximity to newly discovered groups with white, black, and Native American ancestry who also seemed to have been shunned by civilization. In attempting to understand the peculiar conditions of Appalachia, these Progressive Era writers helped to advance some of the first ideas about what it meant to be mixed-race in America.