A 75-year-old male presented with lower back pain, bilateral lower extremity weakness, decreased sensation to vibration and proprioception in lower extremities, anemia, and vitamin B12 deficiency. The MRI of the lumbar spine revealed extensive leptomeningeal carcinomatosis. Subsequently, the patient was diagnosed with multiple myeloma (MM) and B12 deficiency with negative intrinsic factor antibodies. MM can present as extramedullary hematopoiesis (EM) to involve the central nervous system (CNS). CNS involvement is rare and develops in only around 1% of MM patients. It carries a poor prognosis with less than 6 months survival. MM is thought to be associated with both B12 deficiency and pernicious anemia. Some studies have even suggested B12 deficiency as a possible marker for worsening disease and a prognostic factor. In our patient's case, he had extensive CNS involvement at diagnosis of MM with very low B12 levels. The extent of his disease with extensive CNS involvement, which carries a poor prognosis, could possibly explain the very low levels of B12. This is the first reported case of a patient presenting with B12 deficiency found to have MM with leptomeningeal carcinomatosis at diagnosis. To the author's knowledge, there is no literature investigating association between B12 deficiency at the time of diagnosis of MM with CNS complications. Furthermore, there are no established guidelines on treatment for leptomeningeal myelomatosis. We present this case with the effort to learn more about this disease in terms of response and overall survival.