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Sub-Study 2 aimed to investigate how the trajectories of smoking behaviors in developmentally important periods (i.e., adolescence and young adulthood) were associated with subsequent mental health, and to test if alcohol and marijuana use in adulthood might mediate the relationship between smoking trajectories and subsequent mental health. Data were drawn from Round 1 to Round 18 of the National Longitudinal Survey of Youth 1997. Group-based multi-trajectory modeling identified seven distinct smoking trajectories based on longitudinal change in multiple indicators of smoking behaviors over 10 years from adolescence to young adulthood. The results from the linear regression model indicated that late-onset moderate smokers, late-onset accelerated smokers, early-onset heavy smokers, and early-onset moderate smokers showed significantly poorer mental health in later adulthood than stable abstainers, even after controlling for baseline mental health condition and covariates. However, the mental health score of quitters in adulthood was not significantly different from that of stable abstainers. Moreover, the results from the joint significance test and causal mediation analysis demonstrated that the use of alcohol and marijuana in adulthood mediated the association between each smoking trajectory and poor mental health. The findings of sub-study 2 suggest that continued smoking, especially early-onset and heavy smoking, from adolescence to young adulthood may have a long-lasting negative impact on mental health, and quitting may mitigate such impact.