Purpose: Many reports have revealed a high percentage of patients suffering from cognitive impairment due to COVID-19. We investigated cognitive functions using the Montreal Cognitive Assessment (MoCA) in mild COVID-19 patients compared with non-COVID-19 patients within 5 days after the initial respiratory symptoms of infection. Materials and Methods: 113 patients with mild COVID-19 and 109 age- and sex-matched controls were enrolled in this prospective cross-sectional study. Cognitive functions were evaluated using the MoCA. Results: We found that the MoCA scores were significantly lower in the COVID-19 patients than in the controls (P<0.001). Compared to the controls, we found point deficits within some MoCA items such as visuospatial (P<0.001), memory (P=0.017), and attention (P<0.048) domains. In the multivariate model, female sex (2.06 [1.02–4.16], P=0.044), low education level (150.05 [5.16–43.90], P<0.001), high fasting blood glucose level (0.98 [0.96–1.00], P=0.043), and the presence of COVID-19 (24.24 [9.52–61.72], P<0.001) were independently associated with cognitive impairment (OR, 95% CI). Conclusion: We detected cognitive dysfunction, involving the visuospatial domain, memory, and attention, during the acute phase in patients with mild COVID-19.