BackgroundThere is a growing momentum around the world to foster greater opportunities for the involvement of mental health service users in their care and treatment planning. In‐principle support for this aim is widespread across mental healthcare professionals. Yet, progress in mental health services towards this objective has lagged in practice.ObjectivesWe conducted a systematic review of quantitative, qualitative and mixed‐method research on interventions to improve opportunities for the involvement of mental healthcare service users in treatment planning, to understand the current research evidence and the barriers to implementation.MethodsSeven databases were searched and 5137 articles were screened. Articles were included if they reported on an intervention for adult service users, were published between 2008 and October 2023 and were in English. Evidence in the 140 included articles was synthesised according to the JBI guidance on Mixed Methods Systematic Reviews.ResultsResearch in this field remains exploratory in nature, with a wide range of interventions investigated to date but little experimental replication. Overarching barriers to shared and supported decision‐making in mental health treatment planning were (1) Organisational (resource limitations, culture barriers, risk management priorities and structure); (2) Process (lack of knowledge, time constraints, health‐related concerns, problems completing and using plans); and (3) Relationship barriers (fear and distrust for both service users and clinicians).ConclusionsOn the basis of the barriers identified, recommendations are made to enable the implementation of new policies and programs, the designing of new tools and for clinicians seeking to practice shared and supported decision‐making in the healthcare they offer.Patient or Public ContributionThis systematic review has been guided at all stages by a researcher with experience of mental health service use, who does not wish to be identified at this point in time. The findings may inform organisations, researchers and practitioners on implementing supported decision‐making, for the greater involvement of people with mental ill health in their healthcare.