ABSTRACT: INTRODUCTION:Successful competitive employment has been found to be related to enhanced self-esteem, higher quality of life and reduced mental health service use for individuals living with serious mental illnesses (SMIs) including schizophrenia, bipolar disorder and major depression. The effectiveness of the individual placement and support model has been demonstrated in multiple randomised controlled trials in many countries. The management of stress, depression and anxiety in the workplace may be effectively enhanced through digital mental health interventions. The WorkingWell mobile support tool ('app') is specifically designed to meet the need for illness management support for individuals with SMI in the workplace, as an adjunct to professional treatment. METHODS AND ANALYSIS:The WorkingWell app, grounded in evidence-based supported employment, is informed by user experience design. It will be tested in a pre-post design, mixed-methods pilot study to explore issues of feasibility, acceptability and usefulness, and to provide preliminary data on the impact of use. Putative mediators of improved job tenure and psychological well-being, including postintervention changes in social support, self-efficacy and work-related motivation, will be investigated. Forty individuals at least 18 years of age, meeting the eligibility requirements for supported employment services (ie, diagnosed with a mental illness meeting the criteria for severity, duration and treatment), working a minimum of 10 hours per week at study enrolment, and speaking, reading and writing in English will be recruited for the pilot study. Research staff will recruit individuals at community-based mental health agencies; provide orientation to the study, the study smartphones and the WorkingWell app; conduct research interviews including standardised measures as well as semistructured items; and provide technical assistance in telephone calls and inperson meetings. A sample of 10 agency staff will be recruited to obtain further information on the feasibility, acceptability and usefulness of WorkingWell. ETHICS AND DISSEMINATION:The study design and procedures are approved by the Dartmouth-Hitchcock Medical Center Committee for the Protection of Human Subjects, the Massachusetts Department of Mental Health Central Office Research Review Committee and the Vermont Agency of Human Services Institutional Review Board. Study findings will be disseminated to agency partners, state agencies and funders, and to the research and technology development communities. Findings from the study will inform the design, data collection procedures and protocol for future full-scale randomised controlled trial testing of the effectiveness of the WorkingWell app, as well as investigations of work-related variables as mediators of psychological well-being and quality of life for individuals with SMI.