Globally, viral infections substantially contribute to cancer development. Oncogenic viruses are taxonomically heterogeneous and drive cancers using diverse strategies, including epigenomic dysregulation. Here, we discuss how oncogenic viruses disrupt epigenetic homeostasis to drive cancer and focus on how virally mediated dysregulation of host and viral epigenomes impacts the hallmarks of cancer. To illustrate the relationship between epigenetics and viral life cycles, we describe how epigenetic changes facilitate the human papillomavirus (HPV) life cycle and how changes to this process can spur malignancy. We also highlight the clinical impact of virally mediated epigenetic changes on cancer diagnosis, prognosis, and treatment.