We describe the successful use of electroconvulsive therapy (ECT) in a 39-year-old woman with a complex clinical picture including bipolar disorder with catatonic features superimposed on a history of multiple congenital anomalies and intellectual and developmental disabilities. At age 34, the patient presented with marked regression and behavioral disturbance diagnosed as encephalopathy. Multiple medical evaluations were inconclusive, and treatment courses with antibiotics and steroids failed. After psychiatric evaluation, her condition was diagnosed as bipolar disorder with catatonic features. She was treated with ECT with dramatic improvement and resumption of her premorbid level of functioning. We discuss the relevant literature on the use of ECT in patients with intellectual disability.