1997–2010: The District of Lisburn wards of Ballinderry, Ballymacash, Ballymacbrennan, Ballymacoss, Blaris, Derryaghy, Dromara, Drumbo, Dunmurry, Glenavy, Harmony Hill, Hilden, Hillhall, Hillsborough, Knockmore, Lagan Valley, Lambeg, Lisnagarvey, Maghaberry, Magheralave, Maze, Moira, Old Warren, Seymour Hill, Tonagh, and Wallace Park, and the District of Banbridge wards of Dromore North, Dromore South, Gransha, and Quilly.
2010–2024: The City of Lisburn wards of Ballinderry, Ballymacash, Ballymacbrennan, Ballymacoss, Blaris, Dromara, Drumbo, Harmony Hill, Hilden, Hillhall, Hillsborough, Knockmore, Lagan Valley, Lambeg, Lisnagarvey, Maghaberry, Magheralave, Maze, Moira, Old Warren, Seymour Hill, Tonagh, Wallace Park, and part of Derryaghy; the District of Banbridge wards of Dromore North, Dromore South, Gransha, and Quilly and the Ballynahatty and Edenderry parts of the Belvoir ward of the City of Belfast.
2024–present: The District of Armagh, Banbridge and Craigavon wards of Aghagallon, Donaghcloney, Dromore, Gransha, Magheralin, and Quilly; and the District of Lisburn and Castlereagh wards of Ballinderry, Ballymacash, Ballymacbrennan, Ballymacoss, Blaris, Dromara, Harmony Hill, Hilden, Hillhall, Hillsborough, Knockmore, Lagan, Lagan Valley, Lambeg, Lisnagarvey, Maghaberry, Magheralave, Maze, Moira, Old Warren, Ravernet, Wallace Park, and White Mountain.
The seat was created in 1983, as part of an expansion of Northern Ireland's constituencies from 12 to 17, and was predominantly made up from parts of South Antrim and North Down. In their original proposals, in January 1980, the boundary commission proposed calling it 'Lagan'. In further revisions in 1995 it lost some areas to both Belfast West and Strangford.
Following their review of all parliamentary seats in Northern Ireland prior to the 2010 United Kingdom general election the Boundary Commission for Northern Ireland made alterations to Lagan Valley. In an unprecedented decision, passed by Parliament through the Northern Ireland Parliamentary Constituencies Order,[3] one electoral ward was split between two constituencies. This followed concerns in Derriaghy about being moved into the neighbouring but republican West Belfast seat.