Adverse childhood experiences (ACEs) are associated with numerous risk behaviors and mental health outcomes among youth. This study examines the relationship between the number of types of exposures to ACEs and risk behaviors and mental health outcomes among reservation-based Native Americans. In 2011, data were collected from Native American (N = 288; 15-24 years of age) tribal members from a remote plains reservation using an anonymous web-based questionnaire. We analyzed the relationship between six ACEs, emotional, physical, and sexual abuse, physical and emotional neglect, witness to intimate partner violence, for those <18 years, and included historical loss associated symptoms, and perceived discrimination for those <19 years; and four risk behavior/mental health outcomes: post-traumatic stress disorder (PTSD) symptoms, depression symptoms, poly-drug use, and suicide attempt. Seventy-eight percent of the sample reported at least one ACE and 40 % reported at least two. The cumulative impact of the ACEs were significant (p < .001) for the four outcomes with each additional ACE increasing the odds of suicide attempt (37 %), poly-drug use (51 %), PTSD symptoms (55 %), and depression symptoms (57 %). To address these findings culturally appropriate childhood and adolescent interventions for reservation-based populations must be developed, tested and evaluated longitudinally.