Abstract Background Social inclusion is poor among people with mental illness (MI). In recent decades, many countries have developed policies to address this issue. It remains unclear, however, whether their social inclusion has improved over time. This study aimed to assess the evolution of the social inclusion of adults with moderate and severe MI compared to the general population without MI between 1997 and 2018 in Belgium. Methods Data on the general adult population were retrieved from the Belgian Health Interview Survey in six cross-sectional waves from 1997 to 2018. Three degrees of MI severity were compared using the 12-items General Health Questionnaire: no MI, moderate MI, and severe MI (score < 4, 4-7, and > 7). Social inclusion was measured using indicators relating to employment, income, social contacts, and partnership. Results Since 1997, the probability of being unemployed, having limited social contacts, and living on less than 60% of the median national income has been increasing among people with severe MI. Between 1997 and 2018, social inclusion increased among the general population without MI and among people with moderate MI, but decreased among people with severe MI. Conclusions The gap between the social inclusion of people with severe MI and people with moderate or no MI has widened over time, despite major reforms of mental health care and policies. Policymakers and clinical practitioners should pay more attention to supporting the social inclusion of people with more severe MI, particularly in relation to employability and social support. Key messages • In Belgium, the social inclusion of people with severe mental illness has decreased over the past two decades. • The widespread social exclusion of people with severe mental illness is in conflict with international human rights conventions.