The seat was re-created in 1950 when the old Down two MP constituency was abolished as part of the final move to single member seats. Originally the seat consisted of most of the mid and southern parts of County Down, with the north included in North Down. Of the post 1973 districts, it contained all of Down and Banbridge, together with parts of Newry and Mourne, Ards and Craigavon.
In 1983 the seat was radically cut down as part of an expansion of Northern Ireland's constituencies from 12 to 17. Significant parts of the constituency were transferred to either Upper Bann or Newry and Armagh.
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. It still consists of parts of Down, Banbridge and Newry and Mourne districts.
In 2005, the Boundary Commission published provisional recommendations for modifying the boundaries of constituencies in Northern Ireland. For South Down, it originally proposed to add part of Newry from Newry and Armagh and the Loughbrickland part of Banbridge district from Upper Bann, while losing some more of Down to Strangford. These changes were challenged in a round of public consultations, with the result that revised recommendations were made. Under the new proposals, the Newry area will remain in Newry and Armagh and Loughbrickland in Upper Bann. This means that only 4 wards around the town of Ballynahinch were transferred to Strangford. These changes became the final recommendations and were given legal effect in 2008.
The constituency was a predominantly Nationalist area in 1918. The Unionists had significant but minority support. The Sinn Féin candidate polled poorly, probably due to the limited electoral pact to avoid seriously splitting the anti-unionist vote in seats the unionist candidate might have otherwise won.