Limited research has examined the active ingredients and mechanisms of change of naturalistic developmental behavioral interventions (NDBIs). The present study used an exploratory sequential mixed-methods design to develop a comprehensive Theory of Change of Project ImPACT, an empirically supported NDBI. We used qualitative data from interviews with intervention experts (n = 10), community providers (n = 22), and caregivers (n = 12) to develop a comprehensive causal model of the intervention process. We then tested select paths of the causal model using path analyses with an archival dataset (n = 92). The causal model described how developmental techniques aimed at supporting children's attention and engagement lay the foundation for more adult-directed learning opportunities and subsequent child skill growth. However, hypothesized causal relationships were not supported by our quantitative analyses. In the future, this research can be used to develop and prioritize nuanced research questions related to the timing, optimization, and mechanistic process underlying NDBIs.