Background: The aim of this study was to evaluate the effect of an Assertive Community Treatment (ACT) program on psychiatric symptoms, global functioning, life satisfaction, and recovery-promoting relationships among individuals with mental illness. Methods: Participants were patients at the Suwon Mental Health Center. Thirty-two patients were part of the ACT program and 32 patients matched for age, sex, and mental illness were in a standard case-management program and served as a control group. Follow-up with patients occurred every 3 months during the 15 months after a baseline interview. Participants completed the Brief Psychiatric Rating Scale (BPRS), Global Assessment of Functioning (GAF) Scale, Life Satisfaction Scale, and Recovery-Promoting Relationship Scale (RPRS). Results: No significant differences were noted in the sociodemographic characteristics of the ACT and the case-management group. According to the BPRS, the ACT group showed a significant reduction in symptom severity, but the ACT program was not significantly more effective at reducing psychiatric symptoms from baseline to the 15-month follow-up compared to the case-management approach. The ACT group showed more significant improvement than the control group in terms of the GAF Scale. Both groups showed no significant differences in the change of life satisfaction and in the change of recovery-promoting relationships. We observed a significant increase in recovery-promoting relationships in the control group, but the degree of change of recovery-promoting relationships through time flow between groups was not significantly different. Discussion: In this study, we observed that ACT was significantly better at improving the GAF than case management and that participation in ACT was associated with a significant decrease in BPRS scores. However, ACT did not demonstrate an absolute superiority over the standard case-management approach in terms of the BPRS and the measures of life satisfaction and recovery-promoting relationships. Conclusions: ACT may have some advantages over a standard case management approach.