This study experimentally tested risk behavior outcomes of "Connecting," a low-cost, self-directed, family-based prevention program for families with youth placed in their care by state child welfare agencies. Families caring for youth aged 11 to 15 years from across Washington State were recruited and randomly assigned to either the self-directed program with supplemental support (n = 110) or a treatment as usual control condition (n = 110). Program materials included a workbook with family activities and DVDs with video clips. Over the 10-week program, participants received motivational support contacts to prompt program completion. Survey data were collected from youth and their caregivers at baseline, directly following intervention, then again at 12 and 24 months post-intervention. Intervention effects at 24-month follow-up were found to be moderated by age. Among 16- to 17-year-old youth at follow-up, there was an intervention benefit yielding reduced use of any substance (OR = 0.71, 95% CI [0.54, 0.93], p = 0.01) and nonviolent delinquency (OR = 0.73, 95% CI [0.57, 0.94], p = 0.02). There was no intervention effect among adolescents aged 13 to 15 years for any risk behaviors. This evidence suggests that the developmental timing of a self-directed, family-focused preventive intervention for youth and their caregivers in the foster care system may influence risk behaviors that typically emerge in late adolescence. ClinicalTrials.gov Identifier: NCT03157895.