Primary osteoma cutis (cutaneous ossification) is an uncommon disease in which there is bone formation within the skin in the absence of a demonstrable pre-existing condition. Osteoma cutis is a chronic and benign condition. We report a case of a 45-year-old man who developed extramedullary acute leukemia with a myeloid immunophenotype (myeloid sarcoma) with its initial presentation within an isolated pre-existing osteoma cutis in the post-auricular scalp without evidence of systemic acute leukemia or chronic myeloid stem cell disorders. The tumor was surgically excised without complications. Four months later, acute leukemia recurred in the contralateral posterior mandible and showed an immunophenotype consistent with acute lymphoblastic leukemia/lymphoma. The patient now has been treated by standard protocols for acute leukemia. The diagnosis of an extramedullary acute leukemia is challenging because of its inconsistent clinical and histopathologic presentations. Extramedullary acute leukemia developing in a pre-existing osteoma cutis is very unusual and has not been previously reported in the literature.