Malignant melanoma of the lacrimal sac is an exceptionally rare tumor with a poor prognosis. We report two cases of malignant melanoma of the lacrimal sac: a 73 year-old female treated with primary surgical resection and a 75 year-old male treated with surgical resection, adjuvant proton beam radiotherapy, and adjuvant immunotherapy. We discuss the role of post-operative proton beam therapy and recent advancements in immunotherapy. These cases highlight the importance of early diagnosis and multi-modality treatment in this aggressive malignancy.