Background: In December 2019, SARS-CoV-2 spread rapidly worldwide, leading the World Health Organization (WHO) to declare it a pandemic in March 2020. COVID-19 is an infectious disease with symptoms similar to the flu, varying in severity from mild to critical. Post-COVID-19 syndrome refers to the persistence of certain symptoms after recovery from the acute infection. The most common symptoms, such as fatigue, shortness of breath, coughing, and headaches, negatively affect patients’ daily lives. Aim: To investigate the clinical manifestations of post-COVID-19 syndrome concerning the severity of the initial infection. Materials and Methods: A hospital-based cross-sectional study was conducted between 2022 and 2023 at the Pulmonology and Allergy Center of the National Center for Health. In this study, we included participants who came for follow-up one year after being diagnosed with COVID-19. The severity of COVID-19 was categorized according to Living guidance for clinical management of COVID-19 issued by WHO and the COVID-19 diagnostic and treatment guidelines approved by the Minister of Health (A/549). Post-COVID-19 conditions were assessed using the 6-minute walk test, Borg rating of perceived exertion, and Fatigue Severity Scale (FSS). The Chi-square and Mann-Whitney U tests used on SPSS 26.0, with a p-value of <0.05 considered statistically significant. This study was approved by the Research Ethics Committee of the Mongolian National University of Medical Science (2022/03-06). Results: A total of 185 participants were included, comprising 74 men (40%) and 111 women (60%), with a mean age of 57.15±11.29 years. Among them, 90 (48.6%) had severe COVID-19, while 95 (51.4%) had non-severe cases. Clinical symptoms, including cough, dyspnea, fatigue, headache, and myalgia, were statistically different between the non-severe and severe groups. The median FSS score was 45 in the severe group and 38 in the non-severe group, showing a statistically significant difference (p<0.05). The median of 6-minute walking distance was 265 meters in the severe group and 282 meters in the non-severe group (p<0.05). The median Borg score was 2 in the severe group compared to 1 in the non-severe group (p<0.05). Conclusion Individuals who experienced severe COVID-19 exhibited reduced exercise tolerance, increased breathlessness, and more pronounced fatigue than those with non-severe cases.