To date, no single diagnosis has unified the psychiatric illness and the numerous poorly defined physical complaints that Mary Lincoln (née Todd, 1818–1882) suffered in adulthood. Here, I show that her physical ailments spanned 30 years and included sore mouth, pallor, paresthesias, the Lhermitte symptom, fever, headaches, fatigue, resting tachycardia, edema, episodic weight loss, progressive weakness, ataxia, and visual impairment. Long thought hypochondriacal, these findings, plus their time course and her psychopathology (irritability, delusions, hallucinations, with preserved clarity), are all consistent with vitamin B12 deficiency. Pernicious anemia most probably caused this deficiency: she lacked risk factors for other causes, and her consanguineous parents both derived from a region of Scotland having a high incidence of pernicious anemia. A diagnosis of chronic multisystem pernicious anemia would clarify the conduct of Mary Lincoln as First Lady and widow, and illuminate challenges faced by her husband, President Abraham Lincoln. Her case highlights many forgotten features of the natural history of untreated pernicious anemia and is unique in the medical literature in demonstrating such a course extending over a lifetime.