The COVID-19 pandemic and its resulting social restrictions have significant implications for mental health. The objective of this study was to determine the monthly trends and types of patients experiencing a mental health crisis (MHC) who used emergency medical services (EMSs) before and during the COVID-19 pandemic. A retrospective observational study was conducted using EMS data. During the study period, 8577 patients used EMSs for MHCs. EMS dispatches for MHCs and suicide completion after the COVID-19 pandemic were decreased by 12.4% and 12.7%, respectively, compared to those before the COVID-19 pandemic. Segmented regression analysis found that the number of patients per month was 6.79 before the COVID-19 pandemic. The number decreased to 4.52 patients per month during the COVID-19 pandemic, although the decrease was not statistically significant. The monthly number of patients experiencing an MHC decreased during strict social distancing measures but increased during relaxed social distancing measures. The percentage of hanging increased from 14.20% before the COVID-19 pandemic to 14.30% (p = 0.03) during the COVID-19 pandemic, whereas the percentages of jumping (from 15.55% to 15.28%, p = 0.01) and self-harm by smoke (from 4.59% to 3.84%, p < 0.001) during the COVID-19 pandemic were decreased compared to those before COVID-19. However, the effect size for the above findings was small (below 0.20). More than 25% of the patients experiencing an MHC who used EMSs refused to transfer to the ED over both study periods (26.49% in the pre-COVID-19 period and 28.53% in the COVID-19 period). The COVID-19 pandemic and social restrictions seemed to have some effects on the use of EMSs by MHC patients. Hanging is mainly performed indoors and is not found easily if social distancing persists, and a patient experiencing an MHC who refuses to be transferred could potentially attempt suicide. Subsequent studies should be performed to determine whether these findings are temporary during the COVID-19 pandemic or whether they will show different aspects after the COVID-19 pandemic.