Mindfulness training was evaluated as a new treatment for attention and impulsivity problems in adolescents with a variety of different externalizing disorders: attention deficit-hyperactivity disorder, oppositional-defiant and/or conduct disorder, and autism spectrum disorder if characterized by externalizing problem behaviour. It was argued that the large overlap between these three disorders may be partially explained by common underlying attention and behaviour control deficits. Fourteen clinically referred adolescents suffering from externalizing disorders followed mindfulness training in a group format. Parallel, their parents received mindful parenting training. Adolescents and their parents were measured before and after waitlist, after 8-week training, and at 8-week follow-up. No improvement occurred during waitlist on most variables. After mindfulness training, children self-reported substantial improvement on personal goals, internalizing and externalizing complaints, attention problems, happiness, and mindful awareness, and performed better on a sustained attention test. Likewise, parents reported improvement on children's goals, externalizing and attention problems, self-control, attunement to others and withdrawal. In addition, parents improved on their own goals. Improvement was maintained 8 weeks after the training. Consistent with mindfulness theory, increased child awareness after training predicted longer-term improvement in parent-rated child symptoms. Concomitant parent and child mindfulness training appears to be a promising approach for clinic-referred adolescents with attention and impulsivity problems.