High-risk sexual behavior (HRSB) and substance use (SU) are highly prevalent in the general population with adolescents and young adults at high risk for engaging in these behaviors. Unhealthy behavioral patterns established during these developmental periods can have detrimental long-term effects on physical and mental health. Health care expenditures, related to consequences of these behaviors, have been estimated to reach around $740 billion in the United States, indicating an imminent public health concern. Unfortunately, little is known about trajectories and risk factors of health risk behaviors (HRBs) beyond age 25, which is a critical developmental period regarding these behaviors. This study sought to better understand HRB trajectories throughout young adulthood as well as the mechanisms underlying the initiation and progression of these behaviors. This study used data from a large (n = 998), longitudinal, randomized-controlled trial with intensive measurement of HRBs and peer and family processes. Growth mixture modeling estimated gender-specific trajectories of HRSB and SU (tobacco, alcohol, marijuana) from ages 22-30. Multinomial logistic regression (MLR) then examined how family and peer factors, and trauma exposure during adolescence, both separately and in combination, influenced HRB trajectories. Four unique trajectories resulted for SU (low use class; increasing use class; decreasing use class; high use class) and three for HRSB (low HRSB class; increasing HRSB class; deceasing HRSB class). There were no differences in the number of classes or trajectory patterns between men and women. Results of the MLRs demonstrated that deviant peer affiliation (DP), family conflict, parental monitoring and trauma exposure impacted trajectories of tobacco and marijuana use and HRSB during young adulthood, but that the most salient influences were DP and trauma exposure. Alcohol use trajectories and differences between the increasing, decreasing and high trajectory classes for the other HRBs were difficult to predict. These results suggest that young adults are still at risk for engaging in HRBs, and there are risk factors in adolescence that influence typologies of HRBs during this developmental period. Prevention and intervention programs targeting young adulthood are needed, and better understanding factors that lead to vulnerabilities specific to this developmental period may inform targeted interventions.