Hypercalcemic crisis is a rare endocrine emergency that occurs in a small percentage of patients with primary hyperparathyroidism. Although modern diagnostic capabilities allow timely diagnosis and intervention, hypercalcemic crisis still has a high potential for morbidity and mortality. We report an adult patient with a history of kidney stones who presented with heart palpitations, shortness of breath, fatigue, nausea, and 20-pound weight loss over 2 months. Workup revealed hypercalcemia, elevated parathyroid hormone (PTH), and a mediastinal mass on computed tomography angiography. This patient's hypercalcemic crisis was treated with intravenous hydration using normal saline, zoledronate, and furosemide. After medical optimization, the patient underwent definitive treatment with surgical parathyroidectomy via a transcervical approach. At operation, a large parathyroid adenoma was removed from a posterior-superior mediastinal location dorsal to the recurrent laryngeal nerve with subsequent normalization of calcium and PTH. We discuss the timing of parathyroidectomy in cases of hypercalcemic crisis and the surgical approach to ectopic parathyroid glands in the mediastinum.