This dissertation explores the environmental history of the origins of migrant work in the United States between 1870 and 1940. These migrant workers, often called “hobos,” circulated through the Midwest, Great Plains, and Mountain West, providing vital ad hoc labor in agriculture, mining, forestry, and construction. Hobos often worked on any given job for less than a week, never settling in a single place for long. Their itinerancy and homelessness caused them to live outside the law, and they were often forced to steal train rides illegally between jobs, camping alongside the tracks; they were always in danger of being beaten, imprisoned, or turfed to another town by suspicious local police. Totaling somewhere over 100,000 people, mostly white men, they were the single most economically important subgroup of the roughly 1-2 million seasonal workers who lived in the late nineteenth and early twentieth century United States. Hobo labor played a central role in the extractive economy of the American West that was itself critical to the United States’ rise as an industrial power in this period. In this project, I argue that the hobo way of life was the product of two environmental factors: energy and climate. First, I argue that the energy economy of the nineteenth century United States—and even that of the early twentieth century—relied on muscle power for key operations, despite the rising use of steam engines. While Americans harnessed steam for large-scale work, especially in moving freight (railroads) and in providing power for urban industrial operations (factories), medium- and small-scale work required either draft animals (and the men to guide them) or human laborers. Steam engines were simply too bulky for mobile work. But although every part of the country needed muscle power, local environments significantly shaped the use of that muscle power. Second, I argue that the unpredictable climate of the American West made the region’s demand for manual labor likewise unpredictable. Employers in extractive industries hired more or fewer men depending on environmental conditions that could vary from day to day without warning. In doing so, employers took climate risk (e.g., the chance that drought could destroy crops) and shunted it onto their workforce. These migrant laborers therefore lived in a system of climate precarity, with extreme unpredictability in their earnings and day to day lives. This system lasted until the rise of the internal combustion engine, which provided scalable power across the West in the form of tractors and trucks, which could operate closer to the point of extraction than steam engines. Where before, climate risk had been absorbed by hobos, fossil fuels now absorbed much of that risk. Consequently, most migrant labor after 1920 shifted to less ad hoc forms of work—work that could be planned far in advance, allowing for the arrangement of transnational migrant labor from Mexico.