Extracurricular involvement in the school-age years has widespread potential benefits for children's subsequent socioemotional development, especially for low-income youth. However, there is a dearth of research on interventions aimed at increasing school-age extracurricular involvement in low-income youth. Thus, the present study aimed to test the collateral effect of a brief, family-focused intervention for low-income families, the Family Check-Up, on children's school-age extracurricular involvement via improvements in maternal Positive Behavior Support in early childhood. The sample (n = 630, 50% female, 50% White, 28% Black/African American) represented a subsample of families from the Early Steps Multisite Study. At age 2, families were randomly assigned to the Family Check-Up or Women, Infants, and Children Nutritional Supplement Services as usual. Mother-child dyads participated in observed interaction tasks at child ages 2 and 3 that were subsequently coded to assess positive behavior support. Primary caregivers reported on children's school-age extracurricular involvement at ages 7.5, 8.5, and 9.5. Results indicated that although there was not a direct path between intervention status and children's school-age extracurricular involvement, a significant indirect path emerged from intervention group to changes in positive behavior support between ages 2 to 3 to children's school-age extracurricular involvement. The results are discussed in terms of implications for designing preventive interventions in early childhood that promote extracurricular involvement at school-age, particularly for children at risk for maladaptive outcomes.