Mental and cognitive health are key determinants of healthy and successful aging. These two domains are closely intertwined: while chronic stress can impair cognitive functioning, cognitive decline may in turn heighten stress and reduce overall well-being in older adults. This project aims to evaluate the effectiveness and scalability of a multidomain intervention to support mental and cognitive health in older adults. A single-blind, randomized, controlled 2×2 factorial trial will be conducted with a large sample of community-dwelling adults aged 65 years and older in the German- and French-speaking regions of Switzerland. Participants will be randomly assigned to one of four intervention arms: (1) WHO Self-Help Plus stress management training (SH+), (2) multidomain cognitive training focused on lifestyle and behavior change (COG), (3) a combined SH+ and COG intervention (SH+/COG), or (4) an active control group (CG) involving podcast listening and discussion. All interventions will include in-person group sessions, while the SH+, COG, and SH+/COG arms additionally offer a digital platform for self-guided home training. Primary outcomes (mental and cognitive health), secondary outcomes and moderators (cognitive functioning, stigma, self-efficacy, self-concept, loneliness, digital literacy, cognitive reserve, sociodemographic variables) will be assessed at baseline, post-intervention (10 weeks), and at 2- and 6-month follow-ups through a self-administered ambulatory assessment.