With already wide disparities in physical health and life expectancy, COVID-19 presents people with mental illness with additional threats to their health: decreased access to health services, increased social isolation, and increased socio-economic disadvantage. Each of these factors has exacerbated the risk of poor health and early death for people with mental illness post-COVID-19. Unless effective primary care and preventative health responses are implemented, the physical illness epidemic for this group will increase post the COVID-19 pandemic. This perspective paper briefly reviews the literature on the impact of COVID-19 on service access, social isolation, and social disadvantage and their combined impact on physical health, particularly cancer, respiratory diseases, heart disease, smoking, and infectious diseases. The much-overlooked role of poor physical health on suicidality is also discussed. The potential impact of public health interventions is modelled based on Australian incidence data and current research on the percentage of early deaths of people living with mental illnesses that are preventable. Building on the lessons arising from services’ response to COVID-19, such as the importance of ensuring access to preventive, screening, and primary care services, priority recommendations for consideration by public health practitioners and policymakers are presented.