While religion and spirituality (R/S) have been linked to positive mental health outcomes, most studies have employed cross-sectional designs, which do not allow one to evaluate the utility of R/S in predicting these outcomes. To address this gap, this study analyzed data from a 7-year nationally representative, prospective cohort study of U.S. military veterans to examine the effects of R/S on the development of incident mental health outcomes in this population. Specifically, we examined the association between organizational religious activity (ORA), non-organizational religious activity (NORA), and intrinsic religiosity (IR), and the risk of incident PTSD, suicidal ideation (SI), and hazardous drinking (HD). Multivariable logistic regression analyses revealed that ORA predicted a lower incidence of PTSD and SI; NORA a greater risk of developing HD; and IR a lower risk of developing HD. These results suggest that religion and spirituality, assessed in a nationally representative sample of military veterans, predict risk of developing PTSD, SI, and HD, over and above sociodemographic factors and perceived social support. Clinical implications and strategies for incorporating R/S into mental health assessment and interventions in this population are discussed.