Mid Down was a United Kingdom parliamentary constituency in Ireland. It returned one Member of Parliament (MP) to the House of Commons of the United Kingdom from 1918 to 1922, on the electoral system of first past the post.
The constituency was a strongly unionist area, as demonstrated by the small Sinn Féin vote in 1918. Sinn Féin contested the 1918 general election on an abstentionist platform that instead of taking up any seats they won in the United Kingdom Parliament, they would establish a revolutionary assembly in Dublin. All MPs elected to Irish seats were invited to participate in the First Dáil convened in January 1919; in practice Sinn Féin members did so.
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. He did not seek re-election and a new Unionist MP was elected unopposed.
From 1885 to 1918, the parliamentary county of County Down had been divided into four divisions: North Down, East Down, West Down, and South Down. Under the Redistribution of Seats (Ireland) Act 1918, the parliamentary borough of Newry was absorbed into the parliamentary county, which gained the additional division of Mid Down. This division comprised the central northern part of County Down, to the south-east of the city of Belfast. Prior to the 1918 general election, parts of the area had been included in the East Down, West Down and North Down divisions. The area was defined as:
The Government of Ireland Act 1920 established the Parliament of Northern Ireland, which came into operation in 1921. The representation of Northern Ireland in the Parliament of the United Kingdom was reduced from 30 MPs to 13 MPs, taking effect at the 1922 United Kingdom general election. Down was re-established as a two-seat county constituency at Westminster.