There is little information on the mental health needs of adults with intellectual disability (ID) and autism spectrum disorders (ASDs). Such evidence is much needed for the development of more effective mental health services for this group. The aim of this study is to compare adults with ID and ASD receiving specialist mental health services with participants without ASD. Data were collected from the anonymized case records of a clinic-based population in South East London. Health and social functioning were measured using the Health of the Nation Outcome Scale for people with Learning Disability (HoNOS-LD). A review of case records identified 371 service users in receipt of specialist mental health care who were eligible for the study. There were 117 people (32% of the sample) with a clinical diagnosis of ID and ASD. Participants with ASD were younger, more likely to be male, less likely to live independently and had more severe ID than those without ASD. Furthermore, those with ID and ASD were less likely to be diagnosed with a psychiatric disorder but had significantly higher scores on the HoNOS-LD (indicating lower health and social functioning) than those without ASD. A significant proportion of adults with ID who are in receipt of specialist mental health services also have a clinical diagnosis of ASD. This group has different mental health needs compared with those without ASD. The authors note the need for a more personalized approach to service delivery with a focus on improving social functioning and behavioral impairments.