Serious mental illnesses (SMI), including schizophrenia, bipolar disorder, and schizoaffective disorder, are associated with increased medical comorbidity and premature mortality from diabetes and cardiovascular disease. Unhealthy lifestyles, including energy-dense (obesogenic) diet, sedentary behavior, and cigarette smoking are important risk factors for diabetes and accelerated biological aging. All of these risk factors are potentially modifiable. There is considerable literature documenting the effectiveness of strategies to prevent and manage diabetes in the general population; yet, these interventions are rarely offered to people with SMI. Residential Care Facilities (RCFs), called Board-and-Care Homes in California, are a common housing modality for patients with SMI; they provide a venue that can maximize efficiency and sustainability of a lifestyle intervention. The goals of the proposed four-year study are to tailor a multi-component intervention to this high-risk group. The study will be a hybrid effectiveness-implementation (Hybrid Type 1) trial of a Multi-component Intervention for Diabetes risk reduction in Adults with SMI (MIDAS) in licensed RCFs in San Diego county. As a Hybrid Type 1 study, the primary emphasis will be on determining the effectiveness of the intervention to achieve desired health outcomes while also systematically collecting data on its implementation within RCFs that will inform implementation strategy refinement. Main components of MIDAS include: (1) Education about diabetes and lifestyle, (2) Dietary intervention at the facility and resident level, (3) Increased physical activity, and (4) Smoking cessation / reduction. The investigators will employ a modified cluster-randomized stepped wedge and adaptive trial design involving 210 residents with SMI and 120 staff members from 12 RCFs. The RCFs will be divided randomly into four cohorts of three RCFs each. Each cohort will be tested over a 15-month period that includes three phases: a three-month initial control phase (no intervention, from baseline month 0 to end of month 3), a six-month intervention phase (months 4 through 9), and a six-month follow-up phase (no intervention, months 10 through 15). All the study participants will be assessed quarterly during the 15-month period. Our investigators will train RCF staff (especially the Activity Director and cook) to increase physical activity and reduce smoking, and to implement healthful dietary modifications among the residents, using evidence-based interventions. During the intervention phase, the RCF Activity Director will conduct twice-weekly manualized group sessions on education about diabetes, nutrition, exercise, and smoking cessation/reduction, to deliver a multi-component group intervention. We will also explore if there are improvements in blood-based research biomarkers of insulin resistance and inflammation in the RCF residents with SMI. This project is responsive to RFA-MH-17-608, and related to NIMH Strategic Objective #3.3B: testing interventions for effectiveness in community practice settings. If successful, MIDAS will be sustained and disseminated, and would lead to reduction in excess medical comorbidity and mortality associated with SMI.