Background: Little research has been conducted on the mental health needs of adolescents with intellectual disability, despite the severity and rates of such needs being high throughout childhood and in adulthood. We have investigated the prevalence and predictors of mental health needs and service use in adolescents with intellectual disabilities. Materials and Methods: Service-based sample (n = 75) in one catchment area. Individual assessments were carried out. The main outcome was the presence of mental health needs measured by the Developmental Behaviour Checklist. Results: Prevalence of mental health needs increased from 51% as reported by parents to 67% as judged by clinical interviews. Caseness was associated with low adaptive functioning, diagnosis of autism and family history of mental illness. High scores on parent reports of participant mental ill-health showed negative correlations with adaptive functioning scores. Most individuals were in receipt of social and health care. Half of the participants had sought help for mental health needs. Almost half of those receiving medication were on psychiatric medication. Conclusions: Adolescents with intellectual disabilities may have considerable mental health problems which are functionally impairing yet frequently unidentified and hence untreated. Identification of those at risk and undertaking of a comprehensive needs assessment are essential to maximize potential and quality of life and to reduce further deficits and social exclusion.