American immigration law mandates the civil detention of certain classes of migrants while their legal cases proceed through the courts. Due to the peculiar nature of immigration law, many migrants find themselves detained for years on end without receiving the level of due process that normally attends imprisonment. This Note draws on historical and comparative analysis to argue that the mandatory detention provisions of American immigration law are not civil, but functionally criminal, and that detained migrants are therefore owed a modicum of due process that they do not currently receive. This Note traces the history of immigration law in the United States, surveying the laws and cases that gave rise to the mandatory civil detention of certain classes of migrants. This Note then examines recent Supreme Court cases challenging these provisions. Analogizing migrant detention to debtors’ prisons in early modern England and involuntary civil commitment in the substance abuse crisis, this Note identifies four features that help discern civil from criminal detention. Finally, this Note applies those four features to the mandatory detention of migrants under the Immigration and Nationality Act, concluding that this detention is functionally criminal imprisonment, demanding greater due process for migrants.