Research has pointed to both cross-sectional and longitudinal associations between parental internalizing symptoms and child externalizing symptoms. This study analyzed whether the association is mediated by negative parenting behavior in view of previous reports that both parental internalizing symptoms and child externalizing symptoms are related to parenting behaviors. Longitudinal data for the current analyses were derived from a randomized controlled trial on the efficacy of a web-assisted self-help intervention for parents of children with elevated levels of externalizing symptoms. Two different mediation models were analyzed, one using attention-deficit/hyperactivity disorder (ADHD) symptoms as the dependent variable and the other using oppositional defiant disorder (ODD) symptoms. Both models included parental internalizing symptoms as the independent variable, negative parenting behavior as a mediator, and study condition as a confounder. The longitudinal analyses support the mediating role of negative parenting behavior in the association between early parental internalizing symptoms and later child ODD symptoms.