Dual diagnosis clients continue to have low treatment completion rates. The purpose of the current study was to understand if motivational interviewing helped to increase completion rates for clients receiving cognitive behavioral therapy (CBT). Studying the problem was necessary for identifying an evidenced-based model for mental health counselors to help clients with dual diagnoses complete CBT treatment. There were no studies available for understanding the effectiveness of motivational interviewing as a tool for improving treatment completion rates for dual diagnoses clients in intensive outpatient programs. The research question examined if motivational interviewing was effective for improving treatment completion rates for the dual diagnosis population. A quantitative methodology with a quasi-experimental design used for this study and included a paired samples t test, a chi-square test, and a logistic regression analysis. The results showed a statistically significant association between receiving the motivational interviewing techniques and completing CBT. Clients who received motivational interviewing were 4 times more likely to complete CBT treatment compared to clients who did not receive the technique. Clients with increased self-efficacy levels were 2 times more likely to complete treatment, thus addressing the problem of dual diagnosis clients having low treatment completion rates. The overall results demonstrated that clients reduced substance use relapse and recidivism improved. Completing treatment helped to reduce crimes related to drug use; it also prepared substance users for return to society as productive citizens, which promoted positive social change.