OBJECTIVES: People with mental illness may be vulnerable to psychological distress and reduced well-being during the COVID-19 pandemic. The aim of this study was to assess psychosocial and lifestyle predictors of distress and well-being in people with mental illness during the pandemic. METHOD: People with mental illness who participated in an exercise programme prior to the pandemic were invited to complete surveys about mental health and lifestyle corresponding to before and during the pandemic. RESULTS: Social support reduced, alcohol intake increased, and sleep quality and diet worsened during the pandemic, contributing to distress. Psychological distress was associated with the two or more mental illnesses, and negatively associated with having a physical disease. Better diet appeared to protect against increases in distress; loneliness hindered improvements in well-being. CONCLUSIONS: Healthy lifestyle programmes designed to improve social connection may improve health for people with mental illnesses during and after the COVID-19 pandemic.