"Highlights Public health relevance—How does this work relate to a public health issue? • Cancers and mental health conditions are each among the greatest contributors to the global burden of disease; as comorbid conditions, they pose even greater burdens on individuals, families, healthcare resources, and communities. • Complexity across the trajectory of cancer screening and treatment for people with comorbid mental ill-health is a significant and multi-faceted public health issue, underpinned by multiple inequalities in the social determinants of health. Public health significance—Why is this work of significance to public health? • Cancer screening is a pivotal step in addressing cancer burden incidence, mortality, years of life lost, years lived with disability, and disability-adjusted life-years. • Inequity of access to early cancer screening and treatment is understood as a significant contributor to the up to 20-year gap in life expectancy for people with mental ill-health. Public health implications—What are the key implications or messages for practitioners, policy makers and/or researchers in public health? • Access to early and regular screening for cancers is vital to address disparities in health status for people with mental health conditions. • Multiple barriers to cancer screening and treatment at individual, community, health workforce, health service and system levels must be understood and addressed. Abstract The much higher rate of premature mortality from cancer among people with mental ill-health is a major contributor to 20-year reduction in life expectancy for this population, relative to the broader population. Under-screening and delays in screening for cancers are recognised as significant issues contributing to this health inequality. This thematic review explored the common barriers to delayed cancer screening, facilitators to overcome those barriers, and the associated recommendations to improve screening rates for people with mental ill-health. A review of current literature highlighted key themes contributing to the health disparity experienced by this population, including social determinants of health, comorbidities, and health system factors. Facilitators such as trust, support, self-care, and interventions at a health system level were also highlighted. Study quality was appraised using the MMAT v.18 and CASP tools. All studies that met the inclusion criteria, regardless of quality, were included in the review to provide a comprehensive analysis of the existing literature on this topic. Building upon this literature, further recommendations are presented on how to reduce the cancer screening inequality experienced by people with mental ill-health."