Background: The Mental Health Service for People with Learning Disabilities (MHSPLD) is a service development in keeping with UK government policy that promotes cross agency working and access to mainstream mental health services for people with intellectual disabilities. We aimed to show whether the service model brought about improvements in peoples mental state and level of functioning. Method: Community and inpatient groups were compared across three time points using a range of clinical outcome measures that assessed psychiatric symptoms, risk, needs and level of functioning. Results: Inpatients and community groups had similar mental health problems, but inpatients had higher unmet needs and lower functioning, and were at greater risk. There were significant improvements across the range of outcome measures in both groups. Conclusions: Working with mainstream mental health services and across health and social service boundaries delivers effective mental health care for people with intellectual disabilities.