BACKGROUND: Substantial evidence indicates that the COVID-19 pandemic has adversely affected population mental health globally. However, most studies assumed a linear relationship where only negative pandemic-induced changes led to worse mental health outcomes, overlooking the complex relationship between COVID-19-related changes and mental health. This study examined how various types and magnitudes of pandemic-related changes relate to depression and suicidal thoughts in a large, nationwide adult population sample. METHODS: We analyzed data from the 2021 Korean Community Health Survey, a cross-sectional survey of 229,213 adults. The study examined the association between mental health outcomes and three types of pandemic-related changes: daily life impact (scored 0-100), economic activities (employment and income changes), and health behaviors (physical activity, instant food consumption, alcohol consumption, and smoking). Complex sample multiple logistic regression analysis was used to assess these associations, adjusting for sociodemographic factors. RESULTS: The relationship between pandemic-related changes and mental health showed non-linear patterns. Compared to those reporting moderate changes, individuals reporting either no change (depression: aOR 1.253, 95% CI 1.135-1.384; suicidal thoughts: aOR 1.355, 95% CI 1.236-1.486) or complete disruption (depression: aOR 1.895, 95% CI 1.667-2.155; suicidal thoughts: aOR 1.788, 95% CI 1.558-2.052) in daily life showed higher risks of poor mental health. Unexpectedly, positive changes such as improved working conditions (suicidal thoughts: aOR 1.419, 95% CI 1.200-1.677) and increased income (depression: aOR 1.304, 95% CI 1.139-1.493; suicidal thoughts: aOR 1.244, 95% CI 1.079-1.435) were also associated with adverse mental health outcomes. CONCLUSIONS: This study reveals that both minimal and substantial changes in daily life, as well as both positive and negative changes in economic conditions and health behaviors, were associated with poor mental health outcomes during the pandemic. These findings suggest the need for comprehensive mental health interventions that consider various types and magnitudes of life changes during crisis situations.