Background: People living with human immunodeficiency virus (HIV) are at increased risk of developing opportunistic infections and malignancies due to immunosuppression. Among these, Kaposi’s sarcoma (KS) and lymphoma are malignant conditions affecting mucocutaneous tissues and are considered multifocal angioproliferative disorders, as originally described by Moritz Kaposi. These conditions are more likely to occur in individuals with uncontrolled viral replication and low CD4 cell counts. In Mongolia, although the overall prevalence of HIV infection remains low and early initiation of antiretroviral therapy (ART) has reduced the incidence of opportunistic infections, cases of AIDS-related malignancies continue to be reported. Therefore, evaluating the clinical presentation, diagnosis, and treatment outcomes of malignancies in HIV-infected patients is of significant importance. Aim: To present a case of Kaposi’s sarcoma in a patient living with HIV in Mongolia, and to describe the clinical course, diagnostic findings, treatment outcomes, and the impact of antiretroviral therapy (ART). Materials and Methods: This case report is based on a descriptive analysis of a single patient. Clinical data, including medical history, physical examination findings, laboratory parameters (CD4 cell count and viral load), imaging studies, histopathological results, treatment course, and outcomes were collected from the medical records of the National Center for Communicable Diseases and analyzed qualitatively. Case Presentation: A 35-year-old male patient presented with oral lesions that first appeared in June 2010 and progressively increased in number and size. In July 2010, he underwent computed tomography (CT), magnetic resonance imaging (MRI), and histopathological examination in the Republic of Korea, which confirmed the diagnosis of Kaposi’s sarcoma and lymphosarcoma. The patient was enrolled in care at the National Center for Communicable Diseases on September 2, 2010. According to his history, he had lived in Taiwan for the past 10 years and reported having sex with men. On October 22, 2010, his CD4 cell count was 282 cells/ml and viral load was 210,000 copies/ml. Antiretroviral therapy (tenofovir/lamivudine/efavirenz) was initiated. Between 2010 and 2011, he received six cycles of chemotherapy (DOX, CTX, VCR), and surgical excision of skin lesions was performed. Following treatment, the Kaposi’s sarcoma and lymphosarcoma lesions regressed significantly. By August 31, 2012, his CD4 cell count had increased to 535 cells/µL. Conclusion This case demonstrates that HIV-infected patients are at high risk of developing AIDS-defining malignancies due to immunosuppression. Early initiation of antiretroviral therapy (ART) played a crucial role in immune restoration, as evidenced by the increase in CD4 cell count. In addition, combined treatment with chemotherapy contributed to tumor regression and clinical improvement.