Primary osteoma cutis is a rare condition belonging to a spectrum of related genetic disorders, including progressive osseous heteroplasia, plate-like osteoma cutis, and Albright hereditary osteodystrophy, which share identical histologies with cutaneous intramembranous ossification and mutations in GNAS. We report a case of a 15-week-old girl who presented with an enlarging, indurated subcutaneous lesion on her right flank. CT scan showed an extensive subcutaneous sheet of calcification. Histologic evaluation revealed heterotopic calcification and intramembranous ossification within the dermis and mature bone largely replacing the subcutaneous fat compatible with osteoma cutis. Molecular testing was performed and identified an inactivating GNAS mutation. Unique to this case is a dermal proliferation of bland spindle cells that blended with deposited osteoid material. This has not been reported in association with primary osteoma cutis previously. These spindle cells were positive for CD44, Bcl-2, muscle-specific actin, and smooth muscle actin while negative for CD34. We hypothesize that these cells are immature mesenchymal cells, representing an early cellular phase of ossification. We favor these cells provide the background in which ossification is occurring, supporting the theory of osteoblastic metaplasia in the etiology of this condition.