Socially disadvantaged groups are more likely to have poor mental health and experience inequities in mental health service use, quality of care and health outcomes. A promising practice is the development of health equity-focused impact assessment tools that help planners and other people in developing policies, programs and services to identify and address health inequities. This paper presents and reviews Health Equity Impact Assessment (HEIA), a decision support tool developed by the Ontario Ministry of Health and Long-Term Care. The tool has the potential to incorporate health equity into mental health service planning and improve access to and quality of care for marginalized and vulnerable populations.