Objectives: The objectives of this mixed methods study are to (1) investigate the knowledge and skills of mainstream psychiatrists in managing patients with Intellectual Disability (ID) and comorbid mental health disorders, (2) assess their perception of the quality and accessibility of services for this population, and (3) establish the local implementation of the Green Light Toolkit. Method: We surveyed mainstream psychiatrists in the Thames Valley region working in general adult, forensic, and older adult services, to ascertain their opinions about their knowledge and skills in managing patients with ID and comorbid mental disorder, as well as quality and accessibility of services. We compared our findings with previous UK and international research. Results: Respondents mirrored views expressed in earlier studies that inpatient care should be provided in dedicated units for people with ID, rather than general adult inpatient wards. Limited resources, training and competence, and lack of collaborative working were highlighted as key barriers to provision of effective care. Conclusion: Psychiatrists broadly support a specialist service model for people with ID. In the UK, specialist psychiatric services for people with ID have been delivered through inpatient and community services, but there is a current shift towards integrating ID with mainstream service models. Participants expressed concern that mainstream services fail to meet the mental health needs of this patient group, and lead to increased patient vulnerability. The Green Light Toolkit was not well known or used within services. A number of ways of improving collaborative care between services are suggested.