Background: Individuals with mental disorders are known to be socially excluded so that improving social inclusion has become a major goal of healthcare provision. However, empirical research on specific determinants of social inclusion is rather scarce. Methods: A cross-sectional survey of adults with a severe mental illness (n =70) was conducted using a measure of participation and social inclusion for individuals with a chronic mental disorder (F-INK). Univariate and multivariate regression analyses were conducted to identify determinants of social participation and social inclusion. Results: Social participation increased with the number of friends and was, independently thereof, higher in adults living independently than in adults living in supported housing arrangements. The level of social inclusion was higher in those cohabitating and increased with duration of illness. Conclusion: Findings on social participation indicate the need for a re-organization of community-based supported housing arrangements, and, with respect to existing settings, an amendment of present conditions. To promote social inclusion, measures to prime a feeling of ongoing social affiliation should be taken during the first years of psychiatric illness.