The Hydro-Electric Power Commission of Ontario (HEPCO) has been studied as a major factor in the province’s economic development and domestic politics, but its central place in international policy debates has been less recognized. This dissertation argues that the HEPCO should be understood within the contexts of progressivism and transnationalism. The commission was a characteristic product of the international Progressive Era, meant to address fears of monopoly capitalism and social inequalities, alongside regional economic underdevelopment. The HEPCO must also be understood in transnational terms, as it came to influence and was influenced by debates over the public ownership and regulation of electricity in the United States during the Progressive Era. This transnational influence was uneven but cumulative: over time, American progressives became more comfortable and familiar with the commission, and increasingly used it as a model for their policy proposals, just as the private electricity industry regularly vilified it. American progressives’ interest in the commission began in the 1910s and 1920s, aided by classic “vectors of diffusion” such as individual promoters, conferences, and fact-finding trips to Ontario. The “Ontario experiment,” as it came to be known, increasingly served as a model, such as in New York State, where it influenced Franklin D. Roosevelt’s hydroelectricity policies. Eventually Roosevelt’s ambitions surpassed the Ontario model, as his New Deal established the Tennessee Valley Authority and Rural Electrification Administration. A reflexive dynamic can also be observed: Ontarians’ awareness of American interest and opposition to the HEPCO came to influence their political behaviour. In particular, American attention helped contribute to the commission’s independent political activity under Sir Adam Beck. The pluralistic progressivism of the Ontario experiment is also seen in its demobilization during the 1930s, as provincial governments brought the commission under greater control and tamed its populist energies. Today, the progressive legacy of the HEPCO can be observed in the ways it has been publicly commemorated and used in contemporary political debates in Ontario.