PURPOSE: To examine whether adolescents' friends' depressed mood in-school is longitudinally associated with their own mental health from adolescence into adulthood. METHODS: We used data from Waves I-V of the National Longitudinal Study of Adolescent to Adult Health. Participants included 10,769 individuals (Mage = 15.6 [SD = 1.7] years at Wave I, 37.6 years [SD = 1.8] at Wave V; 52% girls; 56% Non-Hispanic White). At Wave I, adolescents nominated friends within their school and reported depressed mood frequency. Friends' depressed mood was the average depressed mood score across adolescents' friends. Using mixed effects logistic regression we examined the association between friends' depressed mood and time-varying outcomes (ego depressed mood, suicidal ideation, suicide attempts) alongside outcomes of lifetime diagnoses of depression and anxiety. We also examined whether associations differed by gender. RESULTS: Adolescents' friends' depressed mood was longitudinally associated with small increases in the odds of their own depressed mood, suicidal ideation, and suicide attempts up to one year later, during adolescence. There was weak evidence this pattern persisted into adulthood or that it was associated with self-reported diagnoses of depression or anxiety. Gender moderated one association: friends' depressed mood was associated with a small increase in the odds of suicidal ideation in boys but not girls. CONCLUSION: Depressogenic friendship networks in adolescence may increase short, but not long-term, risk of depressed mood and suicidality. Findings have important implications for risk assessment, and targeted school-based interventions.