Although parental attention-deficit/hyperactivity disorder (ADHD) is a risk factor for multiple negative youth outcomes, it is unknown how change in parental ADHD symptoms over time affects change in child ADHD symptoms; moreover, mediators of these predictions are largely unknown. Parents of 230 5–10 year-old children (68 % male) with (n = 120) and without ADHD (n = 110) were followed prospectively for 6–7 years across three separate waves. Parents self-reported their ADHD and depression symptoms and similarly rated offspring ADHD, oppositional defiant disorder (ODD), and conduct disorder (CD) symptoms; youth self-reported their substance use. Temporally-ordered mediators consisted of parental expressed emotion (EE), derived from the Five Minute Speech Sample, and self-reported positive and negative parenting behavior. Controlling for key demographics and parental depression symptoms, increasing parental ADHD symptoms were a time-varying predictor of worsening youth ADHD and ODD, although it was unrelated to change in CD and alcohol/substance use. Next, although EE facets (i.e., criticism, emotional over-involvement) did not mediate these predictions, negative parenting behavior significantly mediated predictions of youth ADHD (and marginally in predictions of ODD) from parental ADHD symptoms. These quasi-experimental findings suggest that parental ADHD symptoms are a potential unique causal risk factor for offspring ADHD and ODD; also, preventing negative parenting behavior secondary to parental ADHD symptoms is critical to improve trajectories of youth ADHD and ODD. We consider parental ADHD symptoms and family factors underlying emergent externalizing problems utilizing a developmental psychopathology framework, including implications for intervention and prevention.