Emotion regulation is a mechanism that, when targeted in treatment, can ameliorate heterogeneous psychopathology across ontogeny. Parent-Child Interaction Therapy (PCIT) is one evidence-based intervention hypothesized to improve child emotion regulation. Building on this hypothesis, the present study examined three objectives. First, it examined whether child behavior improvement or parent positive parenting skill use during PCIT predicted improvements in post-treatment child emotion regulation. Second, it measured whether these associations persisted after controlling for baseline child emotion regulation, as well as positive and negative parent emotion socialization strategies. Third, it evaluated whether positive parenting skill use and child behavior improvement mediated associations between pre- and post-treatment child emotion regulation. PCIT participants were 86 2–8 year-olds and their caregivers. Child behavior improvement during PCIT (β = −0.65, p < 0.01) and positive pre-treatment parent emotion socialization strategies (β = −0.33, p = 0.05) were associated with improvements in post-treatment child emotion regulation. Additionally, improvements in child behavior during PCIT (β = −0.29, p < 0.01) but not mid-treatment positive parenting skill use (β = 0.00, p = 0.98) mediated the association between pre- and post-treatment emotion regulation. For children with clinically-elevated emotion regulation scores pre-treatment, over 80% of such scores fell to the normative range post-treatment. These preliminary results indicate PCIT may be effective in improving children’s emotion regulation, such effectiveness may persist even when treatment coincides with parent negative emotion socialization strategy use, and such effectiveness appears attributable to child behavior improvements over treatment. Additionally, parent positive emotion socialization strategies appear to be promising targets for future interventions.