The Missouri Department of Mental Health was first established as a cabinet-level state agency by the Omnibus State Government Reorganization Act, effective July 1, 1974.

State law provides three principal missions for the department: (1) the prevention of mental disorders, developmental disabilities, substance abuse, and compulsive gambling; (2) the treatment, habilitation, and rehabilitation of Missourians who have those conditions; and (3) the improvement of public understanding and attitudes about mental disorders, developmental disabilities, substance abuse, and compulsive gambling.

The seven-member Missouri Mental Health Commission, serves as the principal policy advisory body to the department director. The department of is composed of three divisions: the Division of Behavioral Health, the Division of Developmental Disabilities and the Division of Administration Services, as well as seven support offices.

DMH serves approximately 170,000 Missourians annually through state-operated facilities and contracts with private organizations and individuals. The seven state-operated psychiatric facilities include inpatient psychiatric care for adults and children, as well as sex offender rehabilitation and treatment services. In addition, five habilitation centers and 6 regional offices serve individuals with developmental disabilities. Other services are purchased from a variety of privately operated programs statewide through approximately 2,000 contracts managed annually by DMH.