Multiple environmental risk factors in early childhood predict a broad range of adverse developmental outcomes. However, most prior longitudinal research has not illuminated explanatory mechanisms. Our main goals were to examine predictive associations between cumulative ecological risk factors in early childhood and children’s later externalizing problems and to determine whether these associations were explained by variations in parenting quality. Participants were 241 children (118 girls) at risk for school-age conduct problems and their parents and teachers. Children were approximately 3 years old at Time 1 (T1) and 10 years old at Time 2 (T2). Reports of contextual risk at T1 were used to develop a cumulative risk index consisting of 6 singular risk variables from 3 ecological levels: social resources (low income; social isolation), family resources (marital aggression; poor total family functioning), and maternal resources (single parent status; poor maternal mental health). At T1, parenting variables were measured (corporal punishment, warm responsiveness, maternal efficacy, and negative perceptions of child behavior). At T2, mothers, fathers, and teachers reported child externalizing problems. Johnson’s relative weight analysis revealed that the cumulative risk index was a more powerful predictor of age 10 years externalizing behavior than any of the singular contextual risk variables. Adverse parenting mediated the effects of cumulative risk on later child externalizing problems. Our findings have significant implications for understanding long-term effects of multiple contextual risk factors present in early childhood and for the implementation of positive parenting interventions early on.