John Stuart Mill is central to parallel debates in mainstream contemporary political epistemology and philosophy of federalism concerning the epistemic dimension(s) of legitimate authority. Many scholars invoke Mill to support epistemic arguments for democratic decision-making and decentralized federalism as a means of conferring democratic legitimacy. This article argues that Millian considerations instead provide reason to reject common epistemic arguments for decentralized federalism. Combining Mill's own insights about the epistemic costs of decentralization and recent work in philosophy, politics, and economics undermines (...) purportedly Millian arguments for federalism focused on political experimentation, diversity and participation. Contrary to many interpretations, Millian considerations weaken, rather than strengthen, arguments for federalism. Any valid justification for federalism must instead rest on non-epistemic considerations. This conclusion is notable regardless of how one interprets Mill. But it also supports Mill's stated preference for local decisions subject to central oversight. (shrink)