This study investigated the extent to which parental homework completion during behavioral parent training (BPT) for children with or at risk for developmental delay contributed to parenting and child outcomes. Parents of 48 children (Mage = 44.17 months, SD = 14.29; 73% male; 72% White) with developmental delay (IQ < 75) or at risk for developmental delay (due to premature birth) with co-occurring clinically elevated externalizing behavior problems received Parent-Child Interaction Therapy (PCIT) as part of two previously completed randomized controlled trials. Parental homework completion was measured using parental report of home practice of treatment skills collected weekly by therapists. Parents also reported on child externalizing behavior problems and levels of parenting stress, while parenting skills were observed during a 5-min child directed play and child compliance was observed during a 5-min cleanup situation. Results indicated that higher rates of parental homework completion predicted parenting outcomes (i.e., increased positive parenting skills and decreased levels of parenting stress) and child outcomes (i.e., lower levels of externalizing behavior problems). Additionally, although limited by temporal precedence, there was an indirect effect of reductions in parenting stress on the negative association between parental homework completion and child externalizing behavior problems. These findings highlight the importance of parents practicing skills learned during BPT for optimizing treatment outcome. Parenting stress was also identified as a potential mechanism by which high levels of parental homework completion contributed to reductions in child externalizing behavior problems.