Mast cell sarcoma (MCS) is an extremely rare neoplasm with a clinically aggressive course. Because of its rarity, its morphologic and molecular characteristics are still not well defined. We report a case of a 15-year-old girl with MCS of the temporal bone extending into the posterior fossa creating a mass effect. The lesion mimicked a histiocytic neoplasm morphologically, but showed a novel KIT missense mutation, L799F (2395 C>T). The KIT D816V mutation is frequently found in systemic mastocytosis, but it has not been documented in the few reported human MCS cases. However, 1 reported case of MCS has shown a different alteration in the KIT gene. Our case is the first MCS case with L799F mutation, located between the catalytic loop (790 to 797) and the activation loop (810 to 837) of the KIT gene, and only the second case of MCS with KIT mutation documented in the literature. Proximity of the L799F mutation to the enzymatic region of the KIT tyrosine kinase domain may induce resistance to tyrosine kinase inhibitors.