The authors examined the longitudinal effects of the Family Check-Up (FCU) on parents' positive behavior support and children's school readiness competencies in early childhood. It was hypothesized that the FCU would promote language skills and inhibitory control in children at risk for behavior problems as an indirect outcome associated with targeted improvements in parents' positive behavior support. High-risk families in the Women, Infants, and Children Nutrition Program participated in a multisite preventive intervention study (N = 731) with 3 yearly assessments beginning at child age 2 years. Positive behavior support was measured using 4 indicators derived from at-home observations of parent-child interaction during semistructured tasks. Longitudinal structural equation models revealed that parents in families randomly assigned to the FCU showed improvements in positive behavior support from child age 2 to 3, which in turn promoted children's inhibitory control and language development from age 3 to 4, accounting for child gender, ethnicity, and parental education. Findings suggest that a brief, ecological preventive intervention supporting positive parenting practices can indirectly foster key facets of school readiness in children at risk.