Adolescents in aftercare services who are transitioning from out-of-home care, also called care leavers, face more challenges in their lives, and engage in more risk behaviors, than their peers. However, no previous reviews have comprehensively addressed this issue to identify future research needs. The aim of this systematic review was to gather, assess, and synthesize previous studies concerning care leavers’ high-risk behavior. The search was conducted in six databases, with sixteen articles included in the final review. The selected research highlighted five forms of high-risk behavior: substance abuse, delinquency, sexual behavior, irresponsible use of money, and self-destructive behavior. The incidence of high-risk behavior among care leavers varied noticeably between the studies. Some of the studies reported significant connections between high-risk behavior and gender, race, reason(s) for placement, and the form and number of placements. The synthesized findings revealed a fragmented, limited view of care leavers’ high-risk behavior that highlighted substance abuse and delinquency. The development of adolescents, particularly care leavers, includes multiple factors that have either a conducive or protecting effect for high-risk behavior. Comprehensive research regarding care leavers’ high-risk behavior, including the associated factors, is needed to better support healthy development and success in transitioning to independent living.