Already a rare presentation in patients with systemic mastocytosis (SM), chronic diarrhea is even rarer in those with mast cell leukemia (MCL), a subtype of SM. We present a case to illustrate the utility of lower gastrointestinal endoscopy with histological examination in the diagnosis of MCL in a patient with chronic diarrhea. A woman in her 70s presented with persistent diarrhea. Colonic mucosal biopsy with lower gastrointestinal endoscopy revealed mast cell proliferation, which led to the diagnosis of mastocytosis. Elevated serum tryptase levels and mast cell proliferation in the bone marrow and lymph nodes led to the diagnosis of MCL, a subtype of SM, based on the presence of >30% mast cells in the bone marrow.