People with severe mental illnesses have dramatically reduced life expectancy compared with the general population, which is largely attributed to physical comorbidity. Physical activity and sedentary behaviour interventions offer a safe and viable therapeutic resource for multi-disciplinary mental health care teams. The accumulating evidence supporting the role of these interventions has changed the focus of mental health strategy in some countries, with new developing roles for certain mental health professionals in this field. However, in Ireland the absence of specialised exercise practitioners places a leadership role for mental health nurses in this regard. National mental health strategy in Ireland should prioritise physical activity and sedentary behaviour interventions, make recommendations for the integration of specialised exercise practitioners in all mental health multidisciplinary teams, and recommend the provision of training and awareness for mental health nurses and other multidisciplinary professionals who are already well placed to address this issue.