The effectiveness of an 8-week mindfulness training for adolescents aged 11–15 years with ADHD and parallel Mindful Parenting training for their parents was evaluated, using questionnaires as well as computerized attention tests. Adolescents (N = 10), their parents (N = 19) and tutors (N = 7) completed measurements before, immediately after, 8 weeks after and 16 weeks after training. Adolescents reported on their attention and behavioral problems and mindful awareness, and were administered two computerized sustained attention tasks. Parents as well as tutors reported on adolescents’ attention and behavioral problems and executive functioning. Parents further reported on their own parenting, parenting stress and mindful awareness. Both the mindfulness training for the adolescents and their parents was delivered in group format. First, after mindfulness training, adolescents’ attention and behavior problems reduced, while their executive functioning improved, as indicated by self-report measures as well as by father and teacher report. Second, improvements in adolescent’ actual performance on attention tests were found after mindfulness training. Moreover, fathers, but not mothers, reported reduced parenting stress. Mothers reported reduced overreactive parenting, whereas fathers reported an increase. No effect on mindful awareness of adolescents or parents was found. Effects of mindfulness training became stronger at 8-week follow-up, but waned at 16-week follow-up. Our study adds to the emerging body of evidence indicating that mindfulness training for adolescents with ADHD (and their parents) is an effective approach, but maintenance strategies need to be developed in order for this approach to be effective in the longer term.