Among children with ADHD, parental psychopathology has well-documented, adverse effects on children’s externalizing behavior, yet the underlying predictors of this relation remain understudied. One promising yet untested explanation for the intergenerational transmission of psychopathology is parental cognitive errors (PCEs), which reflect overly negative, stable, and helpless beliefs that parents hold toward their child’s behavior and their own parenting practices. The present investigation examines whether PCEs and parenting behaviors (inconsistent discipline, corporal punishment, and positive parenting) explain the relation between symptoms of parental psychopathology (parental ADHD and depression/anxiety) and child externalizing behaviors in families of children with ADHD (N = 199, ages 7–11). A similar pattern emerged regardless of the type of parental psychopathology examined. PCEs and inconsistent discipline/corporal punishment significantly and collectively accounted for the relation between parental psychopathology symptoms and child externalizing behaviors. No relations were found in the models that examined positive parenting practices. The current findings suggest that addressing parental psychopathology during behavioral parenting interventions, with a particular emphasis on targeting PCEs and their impact on inconsistent discipline and corporal punishment, may hold promise for optimizing treatment response for children with ADHD.