The Australian system of mandatory detention of asylum–seekers has become increasingly controversial. Insofar as commentary on detention has been framed historically, critics have pointed to Australia’s race–based exclusionary laws and policies over the twentieth century. In this article, we suggest that exclusion and detention are not equivalent practices, even if they are often related. Here we present an alternative genealogy of mandatory detention and protests against it. Quarantine–detention and the internment of “enemy aliens” in wartime are historic precedents for the current detention of asylum–seekers. Importantly, in both carceral practices, non–criminal and often non–citizen populations were held in custody en masse and without trial. Quarantine, internment and incarceration of asylum–seekers are substantively connected over the twentieth century, as questions of territory, security and citizenship have been played out in Australia’s histories of detention.