Plenty Mental Hospital was proclaimed as an approved mental hospital under the Mental Health Act 1959 on 19 March 1963. It comprised 12 wards, and was part of the Mont Park Psychiatric Hospital Precinct. The estate also comprised Macleod Repatriation Hospital, Bundoora Repatriation Hospital (VA 5087), Gresswell Sanatorium (VA 5077) and Larundel Mental Hospital (VA 5082).
The Mental Health Act 1959 designated hospitals providing short term diagnosis and accommodations as "psychiatric hospitals". Any institution could have a section designated as a mental hospital for long-term or indefinite hospitalisation and a section designated as a psychiatric hospital for short-term diagnosis and treatment of acute psychiatric illness. Any such designation was published in the Government Gazette. Hence some sections at Plenty Hospital were designated as a mental hospital and some as a psychiatric hospital.
In September 1971, buildings known as Wards C and R were separately proclaimed the Plenty Psychiatric Hospital. In 1980, Ward D and Ward M (rooms 1 and 4) were also removed from the Mental Hospital and added to the Plenty Psychiatric Hospital.
In 1991 Plenty amalgamated with Mont Park and Larundel Hospitals to become part of the North Eastern Metropolitan Psychiatric Services (commonly known as NEMPS) (VA 5083). On 19 August 1991, the proclamation revoking Plenty Mental Hospital as an approved psychiatric hospital was gazetted, and on that date NEMPS took over its administration.