Interventions that focus on the psychiatric and criminogenic needs of justice-involved persons with mental illness are rare. A Treatment Manual for Justice Involved Persons with Mental Illness: Changing Lives and Changing Outcomes (CLCO) was developed specifically for meeting these co-occurring needs. Although results from an initial evaluation indicated that CLCO successfully resulted in reduced symptomatology and some aspects of criminal risk, much additional work examining the effectiveness of CLCO remains to be done. The present evaluation examined the extent to which offenders gained knowledge (i.e., content retention) throughout the program, the extent to which content retention was predictive of program completion, and the extent to which treatment engagement (i.e., session attendance and homework completion) was predictive of program completion. Participants consisted of male and female felony offenders in a residential treatment facility (n = 130), and dually diagnosed male offenders in a residential treatment facility (n = 39). Results indicated that participants in this intervention retained treatment content, and this content retention was predictive of treatment completion. Implications of these findings suggest that CLCO is a promising new intervention for justice-involved persons with mental illness. (PsycINFO Database Record (c) 2019 APA, all rights reserved).