Background: More than one-third of justice-involved individuals meet the criteria for substance use disorder (SUD). Many studies show that treatment expectancy predicts longitudinal SUD outcomes; however, results are inconsistent, and the role of treatment expectancy on SUD outcomes for individuals mandated to a mindfulness-based intervention (MBI) is unknown. Mindfulness-based relapse prevention (MBRP) has shown efficacy with justice-involved populations; however, enrollment in MBRP is typically voluntary. The current study assessed whether pretreatment expectancy predicted SUD- and affect-related outcomes in a sample of women (n = 54) mandated to MBRP as part of their residential SUD programming. Method: The authors employed a quasiexperimental design and administered measures at pre-, mid-, and postcourse. Results: Following mandatory participation in MBRP, significant reductions in craving and substance use were observed. However, contrary to hypotheses, higher pretreatment expectancy predicted greater substance dependence at postcourse. Conclusions: Positive treatment expectancy within the context of an MBI was not related to favorable posttreatment outcomes; in fact, it was related to higher postcourse substance dependence. This suggests that MBIs may be suitable for mandated individuals who may not have voluntarily chosen to participate in such an intervention, and thus may have lower expectancy for the treatment. This finding needs to be replicated in a larger sample to warrant a firmer conclusion.