Seamus Frederick Mallon was an Irish politician who served as deputy First Minister of Northern Ireland from 1998 to 2001 and Deputy Leader of the Social Democratic and Labour Party (SDLP) from 1979 to 2001. He also sat in the House of Commons of the United Kingdom from 1986 to 2005.

The First Minister and deputy First Minister of Northern Ireland are the joint heads of government of Northern Ireland, leading the Northern Ireland Executive and with overall responsibility for the running of the Executive Office. Despite the titles of the two offices, the two positions have the same governmental power, resulting in a duumvirate; the deputy First Minister, customarily spelled with a lowercase d, is not subordinate to the First Minister. Created under the terms of the 1998 Good Friday Agreement, both were initially nominated and appointed by members of the Northern Ireland Assembly on a joint ticket by a cross-community vote, under consociational principles. That process was changed following the 2006 St Andrews Agreement, such that the First Minister now is nominated by the largest party overall, and the deputy First Minister is nominated by the largest party from the next largest community block.