Background Mindfulness interventions with adolescents are in the early stages of development. This study sought to establish efficacy of a mindfulness‐based group intervention for adolescents with mixed mental health disorders. Method One hundred and eight adolescents (ages 13–18) were recruited from community mental health clinics and randomised into two groups (control vs. treatment). All participants received treatment‐as‐usual (TAU) from clinic‐based therapists independent of the study. Adolescents in the treatment condition received TAU plus a 5‐week mindfulness‐training programme (TAU+Mi); adolescents in the control group received only TAU. Assessments including parent/carer reports were conducted at baseline, postintervention and 3‐month follow‐up. Results At postintervention, adolescents in the mindfulness condition experienced significant decrease in mental distress (measured with the DASS‐21) compared to the control group (Cohen's d = 0.43), and these gains were enhanced at 3‐month follow‐up (Cohen's d = 0.78). Overall outcomes at 3 months showed significant improvement for adolescents in the mindfulness condition; in self‐esteem, mindfulness, psychological inflexibility and mental health, but not resilience. Parents/carers also reported significant improvement in their adolescent's psychological functioning (using the CBCL). Mediation analyses concluded mindfulness mediated mental health outcomes. Conclusions Increase in mindful awareness after training leads to improvement in mental health and this is consistent with mindfulness theory. The mindfulness group programme appears to be a promising adjunctive therapeutic approach for clinic‐based adolescents with mental health problems.