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 nine state-operated psychiatric facilities include inpatient psychiatric care for adults and children, as well as sex offender rehabilitation and treatment services. In addition, six habilitation centers and 11 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.