Primary mediastinal B-cell lymphoma (PMBCL) is a rare tumour with different characteristics from other type of lymphomas. Clinical manifestations may vary and cause delay in diagnosis and management. We present a 22-year-old patient with symptoms of shortness of breath, weight loss, and night sweats. Laboratory studies only showed a markedly high lactate dehydrogenase (LDH) level and thoracic computed tomography (CT)scan revealed a large mediastinal mass. Core biopsy-guided CT scan was performed and the pathological and immunohistochemistry established a PMBCL diagnosis. We administered Rituximab Dose-Adjusted Etoposide Prednisolone Vincristine Cyclophosphamide Doxorubicin (R-DA-EPOCH) chemotherapy regimen and the patient responded well to treatment. This is an example of rare case of mediastinal lymphoma with challenges to overcome to achieve diagnostic and therapeutic success. Failure to differentiate PMBCL with other systemic diffuse large B-cell lymphoma (DLBCL) could skew treatment algorithm and prevent optimal response. Administration of proper systemic therapy, especially in young, low-risk patients could yield excellent outcome.