In this study, we assessed value-based decision making in individuals diagnosed with mental illness. Two meta-analyses were conducted of studies that used the Iowa Gambling Task (IGT) to assess value-based decision making. In the first meta-analysis (63 studies, combined N = 4,978), we compared IGT performance in healthy populations and populations with mental illness. In the second meta-analysis (40 studies, combined N = 1,813), we examined raw IGT performance scores as a function of type of mental illness. The first meta-analysis demonstrated that individuals with mental illness performed significantly worse than did healthy control individuals. The second meta-analysis demonstrated no performance differences based on type of mental illness. These findings suggest that value-based decision making is a promising target for transdiagnostic analyses of processes that go awry in mental illness. A critical priority for future work, given that impairment in the IGT could arise from changes in several decision processes, will be to investigate the specific decision processes affected in different mental illnesses.