Using a developmental cascades framework, the current study investigated whether treating maternal depression via interpersonal psychotherapy (IPT) may lead to more widespread positive adaptation for offspring and mothers including benefits to toddler attachment and temperament, and maternal parenting self-efficacy. The participants (N = 125 mother–child dyads; mean mother age at baseline = 25.43 years; 54.4% of mothers were African American; mean offspring age at baseline = 13.23 months) were from a randomized controlled trial of IPT for a sample of racially and ethnically diverse, socioeconomically disadvantaged mothers of infants. Mothers were randomized to IPT (n = 97) or an enhanced community standard control group (n = 28). The results of complier average causal effect modeling showed that engagement with IPT led to significant decreases in maternal depressive symptoms at posttreatment. Moreover, reductions in maternal depression posttreatment were associated with less toddler disorganized attachment characteristics, more adaptive maternal perceptions of toddler temperament, and improved maternal parenting efficacy 8 months following the completion of treatment. Our findings contribute to the emerging literature documenting the potential benefits to children of successfully treating maternal depression. Alleviating maternal depression appears to initiate a cascade of positive adaptation among both mothers and offspring, which may alter the well-documented risk trajectory for offspring of depressed mothers.