How should democratic societies address inequality in an age of plutocratic encroachment and populist indignation? What role should popular movements play in progressive reform efforts? This article turns to the nineteenth-century liberalism of John Stuart Mill for insights on an essential challenge facing democracy today: how to mobilize social movements against intensifying oligarchic threats while safeguarding liberal-democratic values. I advance a novel reading of Mill as a proponent of “liberal plebeianism”—that is, as an activist-theorist who confronted the threat of oligarchy by promoting working-class mobilization within a liberal, parliamentary framework. I trace two discourses within Mill’s writings and speeches: an antioligarchic discourse focused on countering “sinister interests” and a mobilization discourse focused on working-class incorporation. Both follow from Mill’s conviction that liberal reformers should operate as “tribunes of the poor.” This reading helps to clarify Mill’s contested legacy and provides potential resources for understanding how a plebeian orientation might enliven liberal democracy today.