[3] The new building, which was designed by Smyth, Cowser and Partners in the modern style, was opened on 6 July 1970.

[4][5] The design for the seven-storey building involved continuous bands of glazing with concrete panelling above and below.

[7][8] After the county council was abolished in 1973, the building became the regional office of several government departments.

[9][10] The local district of the Northern Health and Social Services Board was located in County Hall but relocated to the former nurses' home on the Route Hospital site in Ballymoney in 1991.

[11][12] Approximately 160 civil servants were still employed by the Northern Ireland Assembly in the building as at 1 January 2014.