A key issue for policy makers and practitioners is finding a way to identify what constitutes a rehabilitative prison. In this study, aspects of a carceral experience that successfully ignite desistance journeys, or that “assist” desistance are identified and measured. We report the findings of a measure to do this, the Macquarie Assisted Desistance Instrument (MADI), which was co-designed with staff and residents in two correctional centers in New South Wales, Australia. Analysis of data from both prison staff and residents (N = 604) revealed that the measure was generally coherent (unidimensional), internally consistent, and stable across time. In addition, those who felt their desistance journeys were being more strongly assisted reported a greater sense of self-efficacy, providing support for the idea that custodial experiences can lead to better rehabilitative outcomes. Suggestions for how to improve prison practice and more meaningfully to assist people to desist from crime are proposed.