Objective: To investigate the relations among certain mental health problems (MHPs; affective, anxiety, attention-deficit/hyperactivity disorder [ADHD], and substance use disorders), criminogenic risk, and outcomes in a sample of serious adolescent offenders. Method: Using data from a longitudinal study of serious adolescent offenders (N = 949; mean age = 16 years, SD = 1.10 years; 84% male; 78% minority), we evaluated the association of MHPs with three distinct outcomes (rearrest, self-reported antisocial activity, and gainful activity), tested whether having an MHP contributed any unique explanatory power regarding these outcomes over and above criminogenic risk markers, and examined whether MHPs moderated the relationship between risk markers and outcomes. Negative binomial and ordinal regressions were used. Data for the study were derived primarily from youth self-report over a 7-year period, with parent collaterals reporting on ADHD, and official records as the source for rearrest information. Results: Of the sample, 57.5% met the criteria for at least one of the assessed MPHs. The presence of a substance use disorder showed consistent associations with the outcomes. After controlling for risk markers and demographic characteristics, MHPs were not associated with most outcomes. The co-occurrence of a substance use disorder and an MHP moderated the relations between several risk markers and outcomes. Conclusions: Current juvenile justice policies that focus treatment efforts on both criminogenic and mental health factors (with particular emphasis on treating substance use disorders) appear to be well founded. It is unlikely that focusing solely on treating MHPs in serious offenders will have a distinct impact on later outcomes. (Contains 4 tables.)