Externalizing behavior problems are among the top mental health concerns in early childhood, and many parenting interventions have been developed to address this issue. To better understand predictors of parenting intervention outcomes in high-risk families, this secondary data analysis evaluated the moderating effect of cumulative risk on child externalizing behaviors, parenting skills, and intervention dropout after completion of a home-based adaptation of the child-directed interaction phase of parent–child interaction therapy (PCIT) called the Infant Behavior Program (IBP). The participants included 58 toddlers (53% male; average age of 13.5 months; and 95% Hispanic or Latine) who were part of a larger randomized control trial in which families were randomly assigned to receive the IBP or treatment as usual (TAU). Cumulative risk was found to moderate the effect of the intervention group on child externalizing behaviors such that the participants in the intervention group with higher cumulative risk scores had greater reductions in externalizing behaviors. A potential explanation for these unexpected findings may be that the obstacles to treatment that were previously imposed by comorbid risk factors (i.e., lack of transportation, time commitment, and language barriers) were adequately addressed such that the families who most needed the intervention were able to remain fully engaged.