In his critique of historicism and utopian social engineering, Karl Popper treats various writers – notably, Plato, Hegel, and Marx – as expounding these mistaken ideas, and as illustrating the threat they pose to ‘the open society’. Among contemporaries, one of those he singles out for criticism is the sociologist Karl Mannheim. While he spends relatively little time discussing Mannheim’s work compared to that of Plato and Marx, I argue that Ideology and Utopia and Man and Society in an Age (...) of Reconstruction may have had particular significance for Popper. I examine his evaluation of Mannheim’s work, assessing its accuracy, and what it reveals about the similarities and fundamental differences in the orientations of these two writers, both of whom had a significant impact upon social science in the twentieth century. (shrink)