Mr Mallon died today at the age of 83.
Born in Markethill in Co Armagh, he was heavily involved in the civil rights movement in the North of Ireland in the 1960s.
He served as the SDLP’s deputy leader of the SDLP from 1979 to 2001.
On news of his passing, Colum Eastwood, the MP for Foyle, this evening described Mr Mallon as a “force of nature”.
“I joined the SDLP because of people like Seamus Mallon,” he said.
“His absolute opposition to the murder and maiming of our neighbours, his immense work to reform policing and deliver a new police service that could command the support of our entire community and his unrelenting commitment to making this a place we can all call home inspired so many young SDLP members.
“Throughout my political life, Seamus was a constant source of guidance, advice and, when needed, some robust critical reflection. His support has been an immense source of personal pride.
“I hope that I’ve done him proud in return.” ”
Mr Mallon served as deputy first minister when Ulster Unionist Party leader David Trimble was First Minister.
Mr Trimble told BBC Radio Ulster that Mr Mallon’s legacy was huge.
“He was committed to politics and he was committed to peaceful democratic politics, that was why he was such a good parliamentarian.
“He was a stubborn man, yes, but at the end of the day he was prepared to make an agreement which we did and we implemented it.
“His legacy will be bound up by the agreement and the institutions and that is something that his family can feel proud of.”
Former British PM Tony Blair said Mr Mallon was “one of the most important architects of peace in Northern Ireland”.
“Brave, blunt, often prepared to swim against the tide if he felt it right, he was someone deeply respected and admired across the troubled landscape of Irish politics,” he tweeted.