Research examining the experience of informal caregivers (ICs) for patients with rare cancers is limited. This was a mixed-methods pilot study of 14 ICs for patients with Erdheim-Chester disease (ECD), an ultra-rare neoplasm. Participants were predominantly female and over half provided at least 60% of their loved one's care. Participants completed measures of the impact of caregiving, caregiver burden, unmet needs, quality of life, anxiety, and depression. Participants reported substantial impact of caregiving, including limiting (50%) or discontinuing (21%) paid employment, and exhausting financial savings (43%). ICs reported a moderate level of burden with five (38%) reporting risk for burnout. While participants reported anxiety (64%) and depression (14%), their overall quality of life was favorable. Semi-structured interviews highlighted factors related to the distress and isolation of navigating a rare cancer. ECD ICs report burden and distress shaped by the experience of providing care for a patient with a rare cancer.