Assertive community treatment (ACT) has been applied to a number of disorders in the adult population, such as schizophrenia, with some degree of success; its use in the treatment of people with intellectual disability (ID) and mental illness has received little attention. Despite the high costs of ID in health and social care, there has been very little evidence-based practice for people with ID and mental illness, and it remains a neglected area of research. The aims of this study were an exploratory comparison of the effectiveness of an ACT model for the treatment of mental illness in people with ID (ACT-ID) with a standard community treatment (SCT-ID) approach. A Randomized controlled trial design was adopted and allocation was made by stratified randomization by an independent statistician. The prognostic factors used in the randomization were gender and psychiatric diagnosis (psychosis vs. affective). Service users were randomly allocated to either ACT-ID or SCT-ID. There were no statistically significant differences between ACT-ID and SCT-ID in terms of the level of unmet needs, carer burden, functioning and quality of life, but borderline evidence of a difference between treatment groups in quality of life in favour of SCT-ID. Both SCT-ID and ACT-ID groups decreased level of unmet needs and carer burden, and increased functioning. SCT-ID also led to a small increase in quality of life.