Maternal depression has been consistently linked to the development of child problem behavior, particularly in early childhood, but few studies have examined whether reductions in maternal depression serve as a mediator in relation to changes associated with a family-based intervention. The current study addressed this issue with a sample of 731 families receiving services from a national food supplement and nutrition program. Families with toddlers between ages 2 and 3 were screened and then randomized to a brief family intervention, the Family Check-Up, which included linked interventions that were tailored and adapted to the families needs. Follow-up intervention services were provided at age 3 and follow-up of child outcomes occurred at ages 3 and 4. Latent growth models revealed intervention effects for early externalizing and internalizing problems from 2 to 4, and reductions in maternal depression from ages 2 to 3. In addition, reductions in maternal depression mediated improvements in both child externalizing and internalizing problem behavior after accounting for the potential mediating effects of improvements in positive parenting. The results are discussed with respect to targeting maternal depression in future intervention studies aimed at improving early child problem behavior.