Neurofibromatosis type 1 (NF1) or von Recklinghausen syndrome is an autosomal dominant disorder that affects the multisystem in the body with complex presentation caused by the neurofibromin gene mutation on chromosome 17. These patients tend to develop soft tissue sarcomas more than the general population. Leiomyosarcoma is a malignant soft tissue tumor that may occur in patients with NF1 in rare cases. We present a case of a rare development of leiomyosarcoma in a 45-year-old female patient with a history of NF1. She developed a progressively growing mass in the left axilla associated with numerous neurofibromas and axillary freckling. MRI revealed a heterogeneous large mixed signal intensity mass in the left axilla, and the diagnosis was confirmed through biopsy.