It returned one Member of Parliament (MP) to the British House of Commons from 1918 to 1922, using the first past the post electoral system .
Prior to the 1918 General Election parts of the area were included in East Down, West Down and North Down constituencies.
The MP elected in 1918, Sir James Craig, became the Ulster Unionist leader and the first Prime Minister of Northern Ireland in 1921.
Having accepted his new office from the Crown on 7 June 1921, Craig's Westminster seat was automatically vacated.
Sinn Féin contested the general election of 1918 on the platform that instead of taking up any seats they won in the United Kingdom Parliament, they would establish a revolutionary assembly in Dublin.