In 1982, Mallon was elected to the new Northern Ireland Assembly, set up as part of then-Secretary of State for Northern Ireland James Prior's rolling devolution. However, due to his membership of the Seanad he was, following a challenge by Unionist politicians, disqualified. Under legislation of the time, no elected member of a British parliament or regional assembly could serve in a parliament outside the United Kingdom or Commonwealth without losing their British seat. That restriction was removed with regard to the Oireachtas by the Disqualifications Act 2000. In 1986, he was elected to Westminster as an MP for Newry and Armagh, a seat he held until 2005. He won the seat in a by-election to replace Jim Nicholson, who had resigned his seat in protest at the Anglo-Irish Agreement, along with all the other Northern Irish unionist MPs. Nicholson was the only MP to fail to be re-elected.
Peace process and 1998 Assembly
Mallon was elected to the Forum for Peace and Reconciliation in 1994. He was a member of the SDLP team at the all-party negotiations that opened in Belfast in June 1996. He has frequently been quoted as saying that the Good Friday Agreement, which resulted from the talks in 1998, was "Sunningdale for slow learners". The Good Friday Agreement led to the setting up of the Northern Ireland Assembly, which was elected in June 1998, with a power-sharing Executive. Mallon was elected as member for Newry and Armagh, and in December 1999 became Deputy First Minister of Northern Ireland, serving alongside Ulster Unionist Party leaderDavid Trimble. Mallon remained a strong opponent of IRA violence, and was also in favour of police reform in Northern Ireland.
Retirement
He retired in 2001, 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 did not contest his seat in the Stormont Assembly in the 2003 elections, and stood down at the 2005 Westminster election. His seat was taken, as expected, by Conor Murphy of Sinn Féin. Mallon was conferred with the freedom of Drogheda in 2018. His autobiography, A Shared Home Place, written with Andy Pollak, was published in 2019.
Personal life
Mallon's wife Gertrude died in October 2016. Their daughter Órla is married with one child. Mallon continued to live in Markethill in retirement. Mallon died at his home in Markethill on 24 January 2020, aged 83. He had been treated for cancer before his death. SDLP Stormont leader, Nichola Mallon paid tribute to Seamus Mallon in the Assembly; describing him as "a man of peace" and "an Irish political giant".