The composition of the seat in 1983 was the entire district of Down, the Annaclone, Ballyoolymore, Croob, Dromore, Drumadonnell, Garran, Quilly and Skeagh, the electoral wards of Banbridge, and the Annalong, Ballycrossan, Binnian, Clonallan, Cranfield, Donaghmore, Drumgath, Kilkeel, Lisnacree, Rathfriland, Rostrevor, Seaview, and Spelga wards from Newry and Mourne.

[5][6] In boundary changes proposed by a review in 1995, the seat was originally to be abolished and replaced by a new Mid Down constituency.

This provoked a storm of protest and following a local enquiry minor changes were made with the seat losing one small section to Lagan Valley and another to Strangford.

Sinn Féin contested the general election of 1918 on a platform that instead of taking up any seats they won in the United Kingdom Parliament, they would establish a revolutionary assembly in Dublin.

In practice, only the Sinn Féin members accepted the offer and their candidate Éamon de Valera only received 0.2% of the votes in South Down, while being elected unopposed for East Clare and East Mayo; the nationalist Jeremiah McVeagh, elected as IPP MP for South Down, did not participate in the First Dáil.

When initially created, this seat had a clear unionist majority, albeit with a strong nationalist minority.

In the October 1974 general election the former Conservative MP Enoch Powell defended the seat for the UUP, representing a coup for them as they gained the support of a high-profile English politician, offering them a spokesperson to the United Kingdom as a whole.

As part of this, he campaigned for the province to have the same ratio of MPs to population as in the rest of the United Kingdom, rather than fewer, which had previously been justified due to the existence of the devolved Stormont Parliament.

Powell was successful in this, but a side effect was that in his own constituency; a significant block of unionist voters were removed, resulting in a nationalist majority.

Powell managed to survive for two election cycles due to a split nationalist vote, but in 1987, he narrowly lost to Eddie McGrady of the Social Democratic and Labour Party, who held the seat until he retired in 2010.