Seamus Mallon was born on in County Armagh. He was educated the Abbey Grammar School in Newry. As a career he chose teaching, becoming headmaster of St. James's Primary School in Markethill. During the sixties he was involved in the civil rights movement, especially in his native Armagh. In 1979, when John Hume went from being deputy leader of the SDLP (under Gerry Fitt) to leader Mallon became deputy leader. He was elected to the first power-sharing Assembly in 1975, and to the Northern Ireland Convention. Between May and December 1982 Mallon was appointed by the then Taoiseach of the Republic of Ireland, Charles J. Haughey to the Republic's upper house, Seanad Éireann. Because of this he was excluded from the then Northern Ireland Assembly, set up as part of then Secretary of State for Northern Ireland James Prior's rolling devolution.1
In 1986 he was elected to Westminster as an MP for Newry and Armagh, a seat he has held since.
Mallon has remained a strong opponent of IRA violence. He has also been in favour of police reform in Northern Ireland. In 1994 he became a member of the Forum for Peace and Reconciliation. Following the Good Friday Agreement in 1998 Mallon became Deputy First Minister in the Assembly, serving alongside Ulster Unionist leader David Trimble. In 2001 Seamus Mallon retired, along with John Hume, from the leadership of the SDLP. Mark Durkan replaced both; Hume as leader and Mallon as Deputy First Minister, when the Northern Ireland Executive was re-established following a suspension.
Mallon has announced that he will not contest his seat in the Stormont Assembly in the 2003 elections. He has given no indication as to whether he will contest his Westminster seat at the next British general election.