Purpose: With the rapid spread of COVID-19 and the restrictions imposed in many parts of the world, there is growing concern about its impact on mental health. This longitudinal study investigated the social participation, social inclusion and psychological well-being in adults with and without mental disorders before the beginning of the pandemic, as well as during and after strict lockdown restrictions in Germany. Methods: The sample (n = 106) consisted of three groups: participants with a chronic mental disorder, with an acute mental disorder, and without a mental disorder at the time of the initial survey. Parameters of interest were assessed using the Measure of Participation and Social Inclusion for Use in People with a Chronic Mental Disorder (F-INK), the Index for the Assessment of Health Impairments (IMET) and the Brief Symptom Inventory (BSI-18). Results: The perceived impairments in social participation and the associated changes in behaviour varied depending on the presence of a mental disorder at the time of the initial survey and were largely temporary, i.e. limited to the period of strict lockdown restrictions. We found no further detrimental effects on mental health 4 weeks after lockdown or later, when a policy of physical distancing was in place. Conclusion: Overall, our findings suggest a general resilience to the official restrictions and measures as well as the pandemic itself. However, further efforts are needed to improve the situation of people with chronic mental disorders and their limited opportunities for social participation.