A few months after the onset of the coronavirus Disease 2019 (COVID-19) pandemic, the worse prognoses of acute myocardial infarction, ischemic and hemorrhagic stroke, and cardiac arrest were reported. This study aimed to investigate the changes in the characteristics and prognoses of these diseases in the emergency department (ED) over a year after pandemic’s onset. This was a retrospective observational study. The year 2019 was defined as the pre-period, while the year from February 2020 to January 2021 was defined as the post-period. Adult patients diagnosed with acute myocardial infarction, ischemic stroke, hemorrhagic stroke, or cardiac arrest during the study period were included. The primary outcome was in-hospital mortality. Time series analyses using autoregressive integrated moving average (ARIMA)(p,d,q) model were performed to evaluate the changes between periods. A multivariable logistic regression analysis of factors affecting in-hospital mortality was performed. The proportions of patients with acute myocardial infarction (0.8% vs. 1.1%, p < 0.001), hemorrhagic stroke (1.0%vs. 1.2%, p = 0.011), and cardiac arrest (0.9% vs. 1.1%, p = 0.012) increased in the post-period. The post-period was independently associated with in-hospital mortality in acute myocardial infarction (adjusted odds ratio (aOR) 2.54, 95% confidence interval (95% CI) 1.06–6.08, p = 0.037) and hemorrhagic stroke (aOR 1.74, 95% CI 1.11–2.73, p = 0.016), but not for ischemic stroke or cardiac arrest. Over a year after onset of the COVID-19 pandemic in Korea, the number of patients with acute myocardial infarction, hemorrhagic stroke, and cardiac arrest in the ED increased. An independent association between the post-period and mortality was observed for acute myocardial infarction, and hemorrhagic stroke. This study provides important information for future studies and policies.