Little is known about the prevalence of intellectual disability (ID) and/or cognitive impairment (CI) among accused persons in the Magistrates (Local) Courts, the personal, health and mental health characteristics of this cohort, and their service provision needs in the community. The study aimed to determine the prevalence of dual diagnoses of ID and/or CI and psychiatric disorder in a sample of accused persons appearing before four Magistrates Courts. Accused persons with ID and/or CI may not be identified in the Magistrates Court as having a disability and therefore may be unable to access the legal safeguards which exist for their protection within the criminal justice system and/or may fail to receive appropriate community health and welfare services.Method The sample was drawn from accused persons aged over 18 years appearing before four Magistrates Courts in metropolitan and urban areas of a large city. Participants were assessed using the Kaufman Brief Intelligence Test, Second Edition (KBIT-2), Vineland Adaptive Behavior Scales, Second Edition (VABS2) and the Psychiatric Assessment Schedules for Adults with Developmental Disabilities Checklist.Results On the KBIT-2, 10% of participants achieved a standard score (SS) below 70 (mild ID range) and a further 20% were in the 70–79 (borderline) range. The VABS2 results indicated that 12% of participants had SS below 70 and a further 9% were in the 70–79 (borderline) range. The prevalence of mental illness in the group with intellectual deficits was 46%, compared with a prevalence of 36% for those without intellectual deficits.Conclusions People with ID and/or CI were found to be over-represented in the Magistrates Court. Furthermore, results highlight the unmet mental health needs of this cohort in the criminal justice system. The results of the study have implications for the planning of services and diversionary options to facilitate better management of defendants with ID and/or CI with mental health needs.