Elevated or abnormal levels of coagulation parameters indicate thrombotic disorders that need to be managed at the earliest to prevent complications and reduce mortality in coronavirus disease (COVID-19) patients. D-dimer is a product that is a part of fibrin abasement, which can be used as a test for the diagnosis of thrombotic disorders. Higher D-dimer levels are a strong indicator of mortality in patients with active COVID-19. This study aimed to determine the relationship between D-dimer levels and the clinical prognosis of COVID-19 patients. This observational, prospective study was conducted between April 2020 and September 2020. All patients with COVID-19 infection were considered for this study, along with their D-dimer values. Patients with higher D-dimer levels and pre-existing morbidities experienced a longer hospital stay and increased complications when compared to patients without pre-existing morbidities. Abnormally elevated D-dimer levels were frequently observed upon admission due to COVID-19 and are analogous to the increased occurrence of critical illness, thrombotic events, acute kidney injury, and death. Therefore, D-dimer levels could be a valuable clinical biomarker and an important indicator of prognosis for patients with COVID-19 with pre-existing morbidities.