People with severe mental illness (SMI) live on average 10 to 20 years less long than the general population. This huge gap is to a large extent explained by an increasing prevalence of cardiovascular disease, type 2 diabetes mellitus and metabolic syndrome. Risk factors include, among others, low levels of physical activity (PA) and sedentary behavior (SB). Increasing PA and reducing SB have a crucial role in preventing the onset of these comorbidities, and thus addressing the mortality gap. There is evidence showing the potential benefit of such interventions for people with SMI. Yet, the translation into clinical practice is slow. A possible explanation are the numerous barriers people with SMI experience to engage in PA. Evidence on the effectiveness of PA and SB interventions is insufficient for policy making. There is also need on information on the value-for-money of such interventions. This evidence can be obtained from health economic evaluation studies. The aim is to co-create a PA and SB intervention that is accessible, engaging and effective for people with SMI by accounting for their needs and preferences, via the Intervention Mapping Protocol and to examine its effectiveness and cost-effectiveness compared to usual care. The involvement of this vulnerable group in the creation process can undoubtedly be considered as innovative. Involving them will help to address and remediate the numerous barriers they experience to engage in PA and reduce SB.