This article is about 1885–1922 constituency of the United Kingdom House of Commons and of the 1919–1921 First Dáil. For the 1923–1969 constituency, see Mayo South (Dáil constituency).
This constituency comprised the southern part of County Mayo. In 1918, the constituency expanded to take in the district electoral divisions of Ballinchalla and Owenbrin from County Galway which had been transferred into County Mayo under the 1898 Local Government Act.
1885–1918: The baronies of Clanmorris and Kilmaine, that part of the barony of Costello contained within the parishes of Aghamore, Annagh, Bekan and Knock, and that part of the barony of Carra contained within the parish of Ballyovey.
1918–1922: The existing South Mayo constituency together with that part of the Connemara constituency contained in the administrative county of Mayo.