Leptomeningeal carcinomatosis is the result of metastatic infiltration of the leptomeninges by malignant cells originating from an extra-meningeal primary tumor site. We describe a patient with active breast cancer who presented with thunderclap headaches (THs) and imaging showing multi-segment irregular arterial narrowing of intracranial vasculature. A 58-year-old Caucasian woman with active stage IV estrogen receptor-positive breast adenocarcinoma and migraine presented with THs. Computed tomography and brain magnetic resonance imaging (MRI) without contrast were unremarkable. Over a period of one week, she had recurrent THs. Interval vessel imaging showed multi-segment irregular arterial narrowing. Treatment with verapamil was initiated for suspected reversible cerebral vasoconstriction syndrome (RCVS). She subsequently had two discrete episodes of confusion with aphasia and left upper extremity numbness. Repeat gadolinium-enhanced MRI showed nodular leptomeningeal enhancement. Lumbar puncture revealed malignant cells in the cerebrospinal fluid consistent with leptomeningeal carcinomatosis. She subsequently underwent whole brain radiation treatment and intrathecal chemotherapy and had no further episodes of TH. Our case emphasizes the importance of considering leptomeningeal carcinomatosis in the differential diagnosis of THs and reversible cerebral vasculopathy, especially in patients with known underlying active cancer. The illustration also proves the importance of a complete work-up in patients with known malignancy in the setting of suspected RCVS.