Background Rates of suicide attempts and nonsuicidal self‐injury [(NSSI); e.g., cutting, burning] peak in adolescence and early adulthood; females and individuals with psychiatric symptoms and diagnoses appear to be at particular risk. Hinshaw et al. [Journal of Consulting and Clinical Psychology, (), 80, 1041] reported that young women with histories of childhood ADHD diagnoses reported higher rates of suicide attempts and NSSI than nondiagnosed, comparison women. Methods Via analyses of an ongoing longitudinal investigation, our aims are to examine, with respect to both aspects of self‐harmful behavior, (a) ADHD subtype differences and effects of diagnostic persistence (vs. transient and nondiagnosed classifications) and (b) potential mediating effects of impulsivity and comorbid psychopathology, ascertained during adolescence. Results Young adult women with a childhood diagnosis of ADHD‐Combined type were at highest risk for suicide attempts as well as the most varied and severe forms of NSSI compared with those with ADHD‐Inattentive type and those in the comparison group; participants with a persistent ADHD diagnosis were at higher risk than those with a transient diagnosis or those never meeting criteria for ADHD. Mediator analyses revealed that, during adolescence, an objective measure of impulsivity plus comorbid externalizing symptoms were simultaneous, partial mediators of the childhood ADHD‐young adult NSSI linkage. Adolescent internalizing symptoms emerged as a partial mediator of the childhood ADHD‐young adult suicide attempt linkage. Conclusions ADHD in females, especially when featuring childhood impulsivity and especially with persistent symptomatology, carries high risk for self‐harm. Psychiatric comorbidity and response inhibition are important mediators of this clinically important longitudinal association. We discuss limitations and implications for prevention and intervention.