The seat was created in boundary changes in 1983, as part of an expansion of Northern Ireland's constituencies from 12 to 17, and was predominantly made up from the old Armagh constituency with the addition of Newry town from the old South Down constituency. In 1995, the Boundary Commission originally proposed to abolish the seat with the Armagh district joining most of Dungannon in a new 'Blackwater' constituency with the rest becoming part of a new Newry and Mourne constituency. This was strongly opposed during the local enquiries and the eventual boundary review left the seat unchanged. It contains the entirety of Armagh district and the Newry half of Newry and Mourne district.
Following the most recent review of parliamentary boundaries in Northern Ireland prior to the 2010 general election the commission originally proposed moving the eastern wards of Newry town to the South Down constituency. This was opposed at the local enquiries and the unchanged Newry and Armagh seat fought at the 2010 UK general election was formed from the following electoral areas:
The entire Armagh government district
The ward of Ballybot, Bessbrook, Camlough, Creggan, Crossmaglen, Daisy Hill, Derrymore, Drumalane, Drumgullion, Fathom, Forkhill, Newtownhamilton, St Mary's, St Patrick's, Silver Bridge, Tullyhappy, and Windsor Hill, from Newry and Mourne district.
The unionist vote in the constituency has declined somewhat in recent years, with the shift being more marked as both the Ulster Unionist Party and the Democratic Unionist Party now stand. The main attention has been upon the rise of the Sinn Féin vote. In the 2001 they surged forward, cutting Mallon's majority drastically, as well as heavily outpolling the SDLP in the equivalent area local elections held on the same day. Then in the 2003 Assembly election Sinn Féin won three seats to the SDLP's one. Mallon stood down at the 2005 general election and, as widely predicted, the seat fell to Sinn Féin.