Background:  Although prior studies have shown that detained females are marked by significant adverse circumstances, little is known about their adult outcomes. Method:  Prospective follow‐up study of 184 (80.4% of original sample of 229) detained adolescent females who were reassessed 4.5 (SD = 0.6) years later in young adulthood (mean age = 20.0, SD = 1.4) on mental health and adjustment outcomes. Associations between these outcomes and detained females’ behavior problems and offense history were examined. Results:  In the total sample, 59.0% had one or more mental health problems at follow‐up, whereas 96.2% were facing at least one adjustment problem. Subjects with a personality disorder (PD) reported more adjustment problems compared to subjects without PD. Mental health and adjustment problems in young adulthood were predicted by detained adolescent females’ behavior problems and offense history. Conclusion:  Detained adolescent females suffered from multiple mental health and adjustment problems in young adulthood. Females who developed PD were most impaired. Results of this study underline the compelling need for continued and gender‐specific interventions. The identification of predictors during detention for poor adult outcomes can serve as targets for intervention.