Previous cross-sectional research has shown that parents of children with attention deficit hyperactivity disorder (ADHD) have high rates of psychopathology, especially ADHD and depression. However, it is not clear whether different types of parent psychopathology contribute to the course and persistence of ADHD in the child over time. The aim of this two wave study was to investigate if mother self-reported ADHD and depression influence persistence of offspring ADHD and conduct disorder symptom severity in adolescents diagnosed with ADHD in childhood. A sample of 143 males with a confirmed diagnosis of ADHD participated in this study. ADHD and conduct disorder symptoms were assessed at baseline and reassessed 4 years later. The boys in this sample had a mean age of 10.7 years at Time 1 (SD 2.14, range 6–15 years) and 13.73 years at Time 2 (SD 1.74, range 10–17 years). Questionnaire measures were used to assess ADHD and depression symptoms in mothers at Time 1. Mother self-reported ADHD was not associated with a change in child ADHD or conduct symptom severity over time. Mother self-reported depression was found to predict an increase in child conduct disorder symptoms, but did not contribute to ADHD symptom levels. This study provides the first evidence that concurrent depression in mothers may be a predictor of worsening conduct disorder symptoms in adolescents with ADHD. It may, therefore, be important to screen for depression in mothers of children with ADHD in clinical practice to tailor interventions accordingly.