A large, nationally representative sample from a preexisting dataset, the National Core Indicators, was used to examine the impact of stress and social support on the mental health of adults with intellectual disability (ID). Stress was significantly correlated with both mental illness and severity of behavior problems, with each additional stressor increasing the odds of poor mental health by 20%. This relationship held, even after controlling for level of ID, gender, and place of residence. Lack of social support was associated with having a mental illness; individuals who lacked social support were twice as likely to have a mental illness. The importance of considering these factors in the prevention, diagnosis, and treatment of mental health in this population is discussed.