AbstractBackgroundPhysical inactivity is a key contributor to chronic disease burden. Physical activity (PA) offers significant health benefits, including for brain health, and to groups with increased dementia risk such as older people with mental illness, Mild Cognitive Impairment or Subjective Cognitive Decline. There is a global call for dementia risk reduction interventions to be implemented into clinical practice. Older people with mental health problems experience significant barriers to PA and are a ‘hard to reach’ group for health promotion. Specific programs, integrated into mental health services, are needed. We will implement behaviour change theories to investigate the effectiveness, acceptability, feasibility and sustainability of an intervention to assist consumers of public Aged Persons Mental Health Services to follow PA clinical guidelines.MethodThis three‐phased study will involve consumers, mental health clinicians and General Practitioners (GPs). An optimisation phase includes initial theory‐based design, using a comprehensive intervention logic model that identifies key anticipated behavioural determinants and matched strategies to support change. The proposed intervention will be presented to consumers and clinicians for consideration. Their feedback will inform tailoring of the intervention prior to implementation. The implementation phase involves a trial of the intervention’s effectiveness compared to usual care for increasing consumer PA levels. Clinicians will be supported to incorporate PA guidelines into clinical practice using tailored behaviour change strategies over 12‐weeks. An individual PA prescription, relevant equipment and exercise demonstrations will be offered to consumers. We will measure PA levels using a validated questionnaire (the CHAMPS) to determine the number of participants meeting PA guidelines, along with measuring changes to quality of life, psychiatric symptom burden and dementia risk. An evaluation phase will use a mixed‐methods approach to evaluate change in the identified key behavioural determinants, as well as qualitative evaluation of the intervention across the three participant groups.ResultsThis study will provide pragmatic evidence‐based information for clinical practice, supporting the design of implementation programs to enhance PA in older consumers with mental health problems.ConclusionThis research addresses the need for integrated mental, cognitive and physical healthcare, facilitating holistic healthcare for groups at increased risk of cognitive decline.