This article examines the process by which British-born migrants to Australia and South Africa were deported from mental hospitals in the 1920s and 1930s. It shows how men and women who arrived as permanent settlers could be re-classified as immigrants subject to expulsion. Debates over who was responsible for those who through mental illness or alcoholism were deemed ‘undesirable’ were conducted at the levels of both high diplomacy and petty bureaucracy. Tracing the history of deportation as a means of social engineering within the empire, this article highlights the tension between the transnational ideology of white supremacy and its expression in national terms. Using the case files of those deported from two settler colonial mental hospitals, Callan Park in Sydney and Valkenberg in Cape Town, as well as official deportation paperwork, it also traces how such diplomatic decisions were refracted through the process of attempted implementation. These files show firsthand both the social history of deportation and the mechanisms through which the settler colonial state aimed to shape its population by excluding not only those perceived to be racially other, but also those judged to be racially unfit. The process of determining domicile and of deportation itself reveals much about the frequently precarious circumstances and life histories of these migrants and their often far-flung networks, as well as the ways in which migrants and their families were able to negotiate the regulatory mechanisms of both the state and the asylum.