This study is a 2-cohort, open-label, phase 1B trial to evaluate the cutaneous toxicity, overall toxicity, and treatment response associated with administering concurrent low-dose TSEB and brentuximab vedotin in patients with mycosis fungoides or Sézary Syndrome. Duration of complete skin response, DOCB, and tumor response in lymph nodes and blood will also be evaluated. Additionally, skin-related QOL and neurotoxicity will be assessed.