To explore the contribution of avoidable mortality to life expectancy inequalities in Wales during 2002-2020. Observational study. Wales, 2002-20, including early data from the COVID-19 pandemic. We used routine statistics for 2002-2020 on population and deaths in Wales stratified by age, sex, deprivation quintile and cause of death. We estimated the contribution of avoidable causes of death and specific age-categories using the Arriaga decomposition method to highlight priorities for action. Life expectancy inequalities rose 2002-20 amongst both sexes, driven by serial decreases in life expectancy amongst the most deprived quintiles. The contributions of amenable and preventable mortality to life expectancy inequalities changed relatively little between 2002 and 2020, with larger rises in non-avoidable causes. Key avoidable mortality conditions driving the life expectancy gap in the most recent period of 2018-2020 for females were circulatory disease, cancers, respiratory disease and alcohol- and drug-related deaths, and also injuries for males. Life expectancy inequalities widened during 2002-20, driven by deteriorating life expectancy in the most deprived quintiles. Sustained investment in prevention post-COVID-19 is needed to address growing health inequity in Wales; there remains a role for the National Health Service in ensuring equitable healthcare access to alongside wider policies that promote equity.