The Kennebec Arsenal Historic District was listed in the National Register of Historic Places on August 25, 1970 in recognition of the site's military and industrial significance. The property was further recognized as a National Historic Landmark on February 16, 2000. Both listings concentrated on the 19th century function of the property as an military outpost. The intention of the recently approved additional documentation was to expand the period of significance and area of significance to reflect the importance of the 20th century use of this property.
In 1903 the Kennebec Arsenal was abandoned by the military and ownership was subsequently transferred from the Federal Government to the State of Maine for use as part of the Maine Insane Hospital. The grounds of the earliest sections of the Maine Insane Hospital are located immediately adjacent to, and south of, the Arsenal complex. The Maine Insane Hospital was established by an act of the State Legislature in 1834 and the first section of the hospital was completed in 1840. The state was faced with a burgeoning need for hospital beds at the end of the 19th century and the facilities at the Arsenal provided ready accommodations. Over the next few years this military campus came to be an integral part of the Maine Insane Hospital grounds.
The building now known as the "Old Max" was constructed between 1907 and 1909 as the Maximum Security Building for the Maine Insane Asylum. The building was conceived of as a unit to house individuals too criminally dangerous to be cared for at the State Hospital and too mentally ill to receive appropriate treatment in the State's penitentiaries. It is an imposing four story brick and granite structure located at the eastern and highest edge of the Kennebec Arsenal parcel. It was designed in an austere Romanesque style by Lewiston Maine architects George M. Coombs and Eugene J. Gibbs, and in design, materials, workmanship, and setting it echoes some of the materials and building massing found in the original Kennebec Arsenal buildings built over sixty years earlier. The Old Max was used as a residential treatment facility through the 1970s, after which it was renovated by the State of Maine to provide office space for the Department of Human Services. In 1983/4 the entry tower was added to provide handicapped access to all floors of the facility.