Objectives: Mental health is the cornerstone of public health, particularly for older adults. There is a clear need to examine the impact of COVID-19 on mental health among older adults in South Korea, where the incidence of COVID-19 was relatively low and widespread transmission was controlled without a national lockdown. Methods: This analysis included a total of 1917 participants from the Experience Survey on Healthcare Use of Older Adults, which was conducted for adults aged 65 years or older by face-to-face interview. Results: The results showed that older adults with a good understanding of COVID-19 public health measures were less likely to experience mental health problems. In contrast, those with a greater risk perception of contracting COVID-19 had higher odds of experiencing tension and anxiety in addition to sadness and depression. Older adults who had a greater fear of COVID-19 and perceived higher-risk of contracting COVID-19 experienced more sleep problems. Conclusion: The findings provide new evidence on the factors that influence the mental health of older adults in South Korea during the pandemic and suggest the development of policy interventions.