Professional mentoring is a specific form of youth mentoring used to promote well-being and prevent adverse outcomes, including delinquency and other forms of antisocial behavior (ASB). Professional mentors are full-time, salaried employees, and the most visible example of professional mentoring is Friends of the Children (FOTC). Youth selected into this program are supported for 12+ years, “no matter what”. Since its inception, FOTC has emphasized the quality of one-on-one, adult-youth relationships as its key change mechanism, often citing research highlighting the importance of one caring adult in the lives of resilient youth. Underappreciated in FOTC’s model of change is research documenting the cascade of factors that contribute to ASB and delinquency, including problematic relationships with parents, teachers, and peers. In this chapter, we present an updated theory of change for professional mentoring, one that reflects developmental processes that contribute to adolescent ASB and appreciates the tenets of prevention science. We first situate the work of professional mentors in the context of extensive research on the causes, course, and consequences of child aggression and later delinquency. Our premise is that professional mentors are uniquely positioned as agents of socialization, capable of providing supportive relationships that span both time and context. Critical to this work is an informed and intentional effort to buffer against the developmental cascade that befalls youth at high risk for concurrent and future problems, and in particular their tendency to affiliate with delinquency-prone peers.