Introduction: Exercise can improve the quality of life (QoL) of people with severe mental illness (SMI); however, many experience practical, psychosocial, and socioeconomic barriers to commencing and maintaining exercise. This protocol describes a study aiming to assess the effectiveness of an Accredited Exercise Physiologist (AEP) service for improving QoL in adults with SMI. Methods and Analysis: This is a prospective non-randomised effectiveness trial to evaluate the effectiveness and cost-effectiveness of an AEP service within mental healthcare settings. Participants will be adults (18+ years) with SMI recruited from public mental health services and community organizations. The primary outcome is QoL assessed using the Recovering Quality of Life questionnaire (ReQoL) assessed at baseline, 3-month and 6-month follow-up. Participants will be invited to a study in which they may self-select to access an AEP service for six months. Self-reported health, demographic and psychosocial influences will be measured and incorporated in analyses as covariates. This is a pragmatic observational approach to evaluating effectiveness, in which participants who attend the AEP service will be compared with those who decline. Reasons for non-participation will be explored, and rates of recruitment and adherence will be examined. Findings will provide insight into implementing AEP services in mental health settings. Ethics and Dissemination: Ethics approval has been obtained from the Metro South Health Human Research Ethics Committee (HREC/2024/QMS/109698). We plan to submit a manuscript of the results to a peer-reviewed journal. Results will be presented at conferences, community and consumer forums and hospital grand rounds. Trial Registration: The trial is registered under the Australian and New Zealand Clinical Trial Registry (ACTRN12624001148561).