More than three decades have passed since adult T-cell leukemia/lymphoma (ATL) was first identified as a distinct disease in Japan. During this time, researchers discovered its causative agent—the human T-cell leukemia virus type 1 (HTLV-1). HTLV-1 infection has a significant global prevalence, affecting an estimated 5–10 million individuals worldwide. In this review, we provide a comprehensive overview of HTLV-1 persistence, latency, replication, and oncogenesis, with a particular focus on the roles of viral proteins such as Tax, HBZ, Rex, p30, p12, and p8. Understanding these mechanisms is essential for developing effective therapeutic strategies against HTLV-1-associated diseases, including ATL. Additionally, we explore current treatment strategies for ATL, including conventional chemotherapy, allogeneic hematopoietic cell transplantation, and epigenetic therapies. Gaining deeper insights into HTLV-1 pathogenesis, its molecular drivers and the existing therapeutic approaches can pave the way for more refined and innovative strategies to combat HTLV-1-associated malignancies.