Children from low socioeconomic status (SES) backgrounds are at particularly heightened risk for developing later externalizing problems. A large body of research has suggested an important role for self-regulation in this developmental linkage. Self-regulation has been conceptualized as a mediator as well as a moderator of these connections. Using data from the Child Development Project (CDP, N = 585), we probe these contrasting (mediating/moderating) conceptualizations, using both Frequentist and Bayesian statistical approaches, in the linkage between early SES and later externalizing problems in a multi-decade longitudinal study. Connecting early SES, physiology (i.e., heart rate reactivity) and inhibitory control (a Stroop task) in adolescence, and externalizing symptomatology in early adulthood, we found the relation between SES and externalizing problems was moderated by multiple facets of self-regulation. Participants from lower early SES backgrounds, who also had high heart rate reactivity and lower inhibitory control, had elevated levels of externalizing problems in adulthood relative to those with low heart rate reactivity and better inhibitory control. Such patterns persisted after controlling for externalizing problems earlier in life. The present results may aid in understanding the combinations of factors that contribute to the development of externalizing psychopathology in economically marginalized youth.