Older adolescents with attention-deficit/hyperactivity disorder (ADHD) have a significantly increased likelihood of developing comorbid depression. It is important to evaluate factors during the early adolescent period that may contribute to this risk. A predominant theory is that impairment and failure experiences lead to the development of low-self efficacy and depression, and that parent and family factors also play a role. In a sample of 326 young adolescents with ADHD (Mage = 12), the present study evaluated whether parent-adolescent conflict mediated the association between social and academic impairment and the development of depression. This study builds upon prior work by evaluating these associations longitudinally and by using a multi-rater approach, including the parent, adolescent, and teacher perspectives. Social and academic impairment directly predicted depression controlling for baseline levels of depression and change in ADHD symptoms. The association between social impairment and depression was partially mediated by parent-adolescent conflict. Mediation through conflict was not found for academic impairment, and the association between academic impairment and depression was no longer significant when accounting for conflict. These findings highlight the importance of social impairment in the development of depression in adolescents with ADHD. Caregivers may play an important role in determining whether adolescents with ADHD internalize social impairment and failure experiences and develop depressive symptoms. Implications of these findings in terms of the importance of interventions focused on parent-adolescent conflict are discussed.