Human T-lymphotropic virus type 1 (HTLV-1) is known to cause a rare form of leukemia/lymphoma called adult T-cell leukemia/lymphoma (ATLL). Although ATLL is known to have a high co-infection rate with human immunodeficiency virus (HIV) in areas where both viruses are endemic, clinical trials, such as the phase three trial for mogamulizumab, continue to exclude patients living with HIV. We here describe the utilization and therapeutic course of mogamulizumab for ATLL in a patient living with HIV. Unfortunately, due to exclusion of patients with co-viral infections in trials, decisions regarding clinical care in these patients remain challenging with the need to rely on retrospective publications for safety and efficacy.