BACKGROUND: The COVID-19 pandemic and its effects have aroused concern all over the world about its impact on mental health. The German National Cohort (NAKO) lends an opportunity to examine trends in altered mental health among adults in Germany during the pandemic. METHODS: We analyzed data from 79 239 participants in the NAKO study who were surveyed with respect to depressive and anxiety symptoms, stress, and self-perceived health at three points in time: at baseline (2014-2019) and in the early (2020) and late (2022) stages of the pandemic. Changes in mental health over time were analyzed with descriptive statistics, Sankey diagrams, and a post-hoc multinomial logistic regression. RESULTS: We observed an initial improvement in self-perceived health from baseline to 2020, but a deterioration from 2020 to 2022. The mental health of most participants remained the same over time. From baseline to 2022, the percentage of participants reporting depressive symptoms rose from 5.9% to 9.7%; rises were also seen in moderate to severe anxiety symptoms, from 3.9% to 6.2%, and in moderate to severe stress, from 4.1% to 10.2%. Both among young adults and among women, deterioration in self-perceived health was more common than improvement. CONCLUSION: Our findings indicate a rise in mental health symptoms among adults in Germany during the pandemic. These findings underscore the need for continuous monitoring and targeted strategies to improve public mental health.