Scholars have extensively studied the diffusion of criminal laws across the American states, and this paper examines an overlooked story of penal diffusion: the mid-twentieth-century spread of habitual offender laws. These laws, which escalated sentences for repeat offenders, proliferated across the states decades before the enactment of the three-strikes laws to which they bore remarkable resemblance. But whereas prior research has traced the legislative diffusion of habitual offender laws, this article alternatively explores how state courts’ interpretations of habitual offender laws diffused across jurisdictions. Using an innovative theoretical framework blending judicial diffusion research with literatures in neo-institutional theory, this article reveals how state courts borrowed legal decisions from other states to interpret, legitimize, and alter laws within their own jurisdictions. This reveals how state courts can shape the trajectory of legislative diffusion in enduring and profound ways. This study’s unique theoretical framework uses the history of habitual offender laws as a case study to explore underappreciated features and dynamics of the diffusion process that have shaped the development of American criminal law.