Introduction Chronic lymphocytic leukemia (CLL) is the most common adult leukemia in Western countries. Extramedullary involvement in the central nervous system (CNS) is a rare complication of the disease, and less than 200 cases have been reported. We report a case of leptomeningeal involvement of CLL that presented as an acute encephalopathy. Case presentation A 76-year-old man with treatment-naïve, Rai stage 0 CLL presented with altered mental status. Cerebrospinal-fluid studies, including flow cytometry, confirmed the leptomeningeal involvement of the previously diagnosed CLL. Surveillance imaging and lab studies showed no evidence of disease progression or Richter's transformation. One-time intrathecal methotrexate resulted in transient improvement of his mental status. Conclusion CLL patients with new-onset neurologic manifestations should be evaluated for the CNS involvement of the neoplasm via brain imaging and cerebrospinal-fluid flow cytometry. This CNS involvement of CLL is associated with poor clinical outcomes. Intrathecal treatment with methotrexate, cytarabine, and steroid may improve neurologic symptoms.