We examined the stability of and cross-influences between externalizing behaviors and intervention engagement among children participating in a randomized clinical trial of an intervention for disruptive behavioral youth. Analyses also accounted for the influence of caregiver depression, family relationship quality, and sociodemographic factors (race, income) on the relationship between behaviors and intervention engagement. Analyses were based on 118 children participating in the Coping Power intervention. Composite variables were created to represent externalizing behaviors and intervention engagement constructs. Associations between these composite variables were examined over 24 treatment sessions. Findings indicated a regressive relationship among externalizing behaviors, i.e., baseline externalizing behaviors were positively associated with immediate follow-up behaviors. There were also dynamic relationships observed among engagement constructs. Notably, engagement with in-session activities during sessions 1–8 was positively associated with out-of-session activity engagement during the same treatment time period. Engagement with out-of-session activities during sessions 1–8 was positively associated with in-session activity engagement during sessions 9–16, indicating a complete mediation between early and middle in-session engagement through the mechanism of early out-of-session engagement. A crosslag relationship was observed: middle in-session engagement was negatively associated with externalizing behaviors at immediate follow-up. Finally, an interaction of race by income on immediate follow-up externalizing behaviors was observed, such that Black children’s externalizing behaviors remain static regardless of income level while White children’s behaviors decreased with higher income. Our findings support the contention that focusing on intervention engagement may be especially important in prevention interventions.