Leiomyosarcoma is a rare type of tumor of smooth muscle cells that can occur anywhere in the body. However, it typically occurs in the retroperitoneum, intra-abdominal sites, and uterus in people over 65. Here is a case of a 71-year-old male with a history of melanoma of the skin who presented with a rapidly enlarging, non-tender lump at his left lateral thigh area that was later diagnosed as pleomorphic dedifferentiated leiomyosarcoma. The patient underwent radical resection of the tumor and the attached vastus lateralis muscle and partial lateral collateral ligament, followed by radiation therapy to the resected site. He had no evidence of tumor recurrence on follow-up imaging for several months until he was found to have metastatic disease to the lungs on a surveillance CT one year later. A biopsy confirmed that the lung nodules were leiomyosarcoma metastases, and the patient was started on chemotherapy and stereotactic body radiation therapy (SBRT). Upon reviewing the literature, a few cases of leiomyosarcoma arising from the thigh muscles were found.