While children and adolescents placed in child welfare or juvenile justice out-of-home care show higher prevalence rates of mental disorders compared to the general population, it remains unclear whether this pattern persists into adulthood. A quantitative synthesis of existing studies is lacking. The aim of this meta-analysis was to estimate the prevalence rates for mental disorders among adults with a foster or residential child welfare or juvenile justice care history, comparing them where possible to rates among the general population. PubMed, PsycInfo, EMBASE, and Web of Science were systematically searched for epidemiological studies published up to 28 October 2020. Nineteen studies, totaling 604,257 participants, met our inclusion criteria. Random-effects models were used for prevalence rates and odds ratios (OR) of mental disorders, and study quality was rated. A prevalence rate of 30% [95% CI (23.36, 37.36)] for any mental disorder in adults with a child welfare care history was found (3–17% for specific disorders). A prevalence rate of 45% [95% CI (42.38, 47.38)] for any mental disorder was found in adults with a juvenile justice care history (6–66% for specific disorders). For out-of-home placement history, adult mental disorders were significantly higher than in the general population (OR = 1.33–2.76). Studies differed in terms of methodology and the disorder groups considered, so heterogeneity between effect sizes ranged from low to high. Our findings suggest that the high risk that mental health issues will persist in adults with an out-of-home placement history needs to be taken seriously in the transition from adolescence to adulthood. The care systems involved need to collaborate and to be aware of these risks.