Limited data are available on lifetime prevalence and age-of-onset distributions of psychological disorders and suicidal behaviors among Army personnel. We used simulation methods to approximate such estimates based on analysis of data from a U.S. national general population survey with the socio-demographic profile of U.S. Army personnel. Estimated lifetime prevalence of any DSM-IV anxiety, mood, behavior, or substance disorder in this sample was 53.1 percent (17.7 percent for mood disorders, 27.2 percent for anxiety disorders, 22.7 percent for behavior disorders, and 14.4 percent for substance disorders). The vast majority of cases had onsets prior to the expected age-of-enlistment if they were in the Army (91.6 percent). Lifetime prevalence was 14.2 percent for suicidal ideation, 5.4 percent for suicide plans, and 4.5 percent for suicide attempts. The proportion of estimated pre-enlistment onsets was between 68.4 percent (suicide plans) and 82.4 percent (suicidal ideation). Externalizing disorders with onsets prior to expected age-of-enlistment and internalizing disorders with onsets after expected age-of-enlistment significantly predicted post-enlistment suicide attempts, with population attributable risk proportions of 41.8 percent and 38.8 percent, respectively. Implications of these findings are discussed for interventions designed to screen, detect, and treat psychological disorders and suicidality in the Army.