Background and Objectives: This study aimed to analyze the prevalence of mental disorders in patients with breast cancer at Ajou University Hospital. In addition, the patterns and prevalence of mental disorders according to the occurrence of coronavirus disease (COVID-19) were analyzed. Materials and Methods: From 1 January 2008 to 30 June 2021, psychiatric disorders were identified in 5174 female patients diagnosed with breast cancer at Ajou University Hospital. Based on the time when COVID-19 occurred, the pattern of onset of mental disorders in patients with breast cancer was analyzed. In addition, the prevalence of mental disorders according to the time of breast cancer diagnosis and age was evaluated. Results: A year before the diagnosis of breast cancer, 371 patients were diagnosed with a mental disorder. Of these, 201 patients were diagnosed with stress and adjustment disorders, and 97 patients had anxiety disorders. The overall frequency of psychiatric disorders after breast cancer diagnosis peaked two months later. Among psychiatric disorders reported before the COVID-19 pandemic, the proportion of stress/adaptation disorders was 52%, and among psychiatric disorders reported after the pandemic, it was significantly higher at 94.7%. Anxiety was found to be high in the elderly group aged ≥ 60 years, and the prevalence of stress and adjustment disorders tended to increase in the non-elderly group. Conclusions: Breast cancer patients showed different patterns of psychiatric disorders according to age, time of breast cancer diagnosis, and the occurrence of COVID-19. Owing to the COVID-19 pandemic, delays in treatment and anxiety about infection have increased the rate of stress and adjustment disorders in cancer patients. Mental health management during the pandemic and after cancer diagnosis can improve the quality of life of patients with cancer.