HIV and other sexually transmitted infections (STIs) are important public health challenges in the US. Adverse childhood experiences (ACEs), including abuse (emotional, physical or sexual), witnessing violence among household members, may have an effect on sexual behaviors, which increase the risk of HIV/STIs. The aim of this study was to examine the sex differences in the role of posttraumatic stress disorder (PTSD), major depression (MD), substance use disorders (SUDs), early sexual debut, and intimate partner violence (IPV) perpetration as mediators in the association between ACEs and HIV/STIs. Data were obtained from Wave 2 (2004–2005) of the National Epidemiologic Survey on Alcohol and Related Conditions. Structural equation modeling was used to determine the role of PTSD, MD, SUDs, early sexual debut, and IPV perpetration as mediators in the relationships between ACEs and HIV/STIs. Differences and similarities existed in the mediational roles of psychopathology and sexual behaviors. For example, among men, MD fully mediated physical/psychological abuse (β = 0.0002; p = 0.012) and sexual abuse (β = 0.0002; p = 0.006), and HIV/STIs while among women, MD fully mediated physical/psychological abuse (β = 0.0005; p < 0.001) and parental violence (β = −0.0002; p = 0.012). Among men, IPV perpetration fully mediated sexual abuse (β = −0.0005; p = 0.012) and HIV/STIs while among women, IPV perpetration was not a statistically significant mediator. HIV/STI prevention and intervention programs should use a life course approach by addressing adverse childhood events among men and women and consider the sex differences in the roles of psychopathology and sexual behaviors.