Persons with mental health diagnoses use tobacco at alarming rates, yet misperceptions remain about the effect of quitting on mental health outcomes. This article examines the relationship between tobacco cessation and changes in severity of mental illness. Participants were N = 577 veterans with a history of mental health treatment enrolled in a tobacco cessation study. The effects of abstinence and time on Behavior and Symptom Identification Scale–24 summary scores and subscales were examined. Abstinence at both 2 and 6 months post-baseline was related (p < .0001) to lower Behavior and Symptom Identification Scale–24 summary scores and improvement on three Behavior and Symptom Identification Scale–24 subscales. Providers should recommend and provide tobacco treatment to all mental health patients to improve their physical and mental health functioning.