Before 1969, the area formed part of the Northern Ireland Parliament constituencies of Mid Down and Iveagh.
Lagan Valley was created by the Electoral Law Act (Northern Ireland) 1968 as a division of County Down.
The seat included Hillsborough, Drumbo and Carryduff as well as the sprawling rural area of Ballymacbrennan, which are split between the Lagan Valley and South Belfast and Mid Down constituencies.
The Parliament was prorogued on 30 March 1972, under the terms of the Northern Ireland (Temporary Provisions) Act 1972.
It was formally abolished in 1973 when the Northern Ireland Constitution Act 1973 received Royal Assent on 18 July 1973.