Individual and group therapy have been found to benefit from the use of client feedback—brief assessments completed by the client each session to track client treatment outcomes and therapeutic alliance. However, the use of client feedback has not been examined for the population of individuals diagnosed with co-occurring mental illness and substance abuse. This mixed-methods study was conducted with adults diagnosed with co-occurring mental illness and substance abuse who attended group therapy at an outpatient day treatment facility. A single-case reversal design was used to determine the effect of using client feedback in group therapy on therapeutic outcomes and attendance. Further, this study applied thematic analysis to interviews conducted with group therapy clients after they experienced the client feedback intervention. Effect sizes were calculated for attendance and outcome scores between each phase. A significant difference in attendance between control and treatment conditions was indicated for the second intervention phase, however no other significant differences were found. A number of themes emerged from thematic analysis of the interviews, and are discussed in their relation to the utility of assessments in therapy and future directions for the field of group therapy. Client characteristics among experimental, interview, and pre-attrition groups and their effect on outcomes are also discussed.