Background: Engagement of individuals with an intellectual disability and their caregivers in health care has been highlighted as an essential factor in developing services that better meet client needs; however, few studies have focused on the engagement of individuals with a dual diagnosis (intellectual disability co-occurring with a mental health illness and/or challenging behaviour) in health care. Methods: This qualitative participatory action study engaged clients and their caregivers in exploring principles and actions that can be applied to build active and meaningful engagement within a specialised community-based mental health service for adults with a dual diagnosis. Results: The four themes that emerged from the data--Collaboration, Communication, Empowering Environment, and Tailored Service--offered a way to conceptualise client and caregiver engagement. Conclusion: This study underscores the importance of building a culture of service delivery that values, encourages, and supports client and caregiver engagement. Practical strategies for increasing engagement are provided.