Mourning and melancholia, one of Freud's 1915 ‘metapsychological papers’, is recognised as containing the origins of contemporary object relations theory. It offered startling new insights into the formation of the mind and initiated a paradigm shift in Freud's metapsychology. The present paper suggests that Mourning and melancholia possessed such transformative power because it represented the culmination of a long period of transformation within Freud himself. Links between Freud's personal experiences of mourning and the evolution of his theory, in particular his metapsychological conceptions, are considered. Three varied experiences of mourning are identified. First, Freud's abandonment, in 1895, of the highly mechanistic and concretized metapsychology of his Project for a scientific psychology. Second, Freud's mourning for the death of his father in 1896, which provided the emotional impetus for his self-analysis and The interpretation of dreams. And third, a period of mourning following the outbreak of the First World War. It is suggested that this experience resonated with his mourning for his father twenty years earlier, leading to a precipitation of insights about the transformative power of mourning, and its fundamental importance. Despite Freud's abandonment of the 1895 Project, subsequent metapsychological reformulations remained characterised by an emphasis on explanatory ‘mechanisms’. In Mourning and melancholia however, new metapsychological formulations emerge out of accumulated clinical observation and inference such that a link is preserved between clinical thinking and metapsychological formulation.