Abstract Multidomain lifestyle interventions can improve cognitive function and mobile health technologies can deliver cost-effective interventions. We developed the smartphone app, Cognitive Evergreenland, to promote cognitive health in people at high risk of dementia, and assessed its usability. Functional modules were selected using a behaviour change wheel (BCW) theory-based method. Target behaviors were assessed by literature review and focus group interviews. Findings were mapped onto the Capability, Opportunity, and Motivation—Behaviour (COM-B) model and Theoretical Domains Framework, identifying behaviors requiring change and linking them to intervention functions. Behavior change techniques (BCTs) considered likely to be effective were selected, and corresponding COM-B components and BCTs translated into application functionalities. The app was optimized based on user feedback collected by interview and evaluated using “Mobile Health App Usability Questionnaire for Standalone mHealth Apps (Patient Version)”. Promoting adherence to multidomain lifestyle interventions required changes in physical and psychological abilities, reflective and automatic motivation reinforcement, and social and physical opportunities provision. We identified seven key intervention functions and selected 16 BCTs. Finalized Cognitive Evergreenland modules included health education, cognitive stimulation, cognitive training, interactive communication, health diary, functional assessment, and personal profile. Target users indicated overall satisfaction with usability. BCW theory application facilitated Cognitive Evergreenland development.