Since the mid-1960s, the carceral population in the US has increased around 900%. This article analyses that increase from a Marxist framework. After interrogating the theories of Michelle Alexander and Loïc Wacquant, I lay out a theoretical framework for a Marxist theory of mass incarceration. I then offer a historical analysis of mass incarceration in keeping with this theoretical framework, emphasising the carceral system’s relationship to the class struggle and the large-scale economic dislocations of post-Fordism. Finally, I emphasise how private prison companies, increasingly central to the story of mass incarceration, are influencing current efforts to reform the prison system and shift to ‘alternatives to incarceration’.