Mill's political and his international theory rest on a philosophy of history drawn in turn from the experience of nineteenth century imperialism. And yet, this philosophy of history remains unexamined in Political Theory and International Relations (IR) alike, largely because of the peculiar division of labour between the two disciplines. In this article I will argue that this omission results not just in a misconception of those aspects of Mill's thought with which Political Theory and IR directly engage; in addition, and more seriously, it has led in both disciplines to an unreflected perpetuation of Mill's justification of imperialism.