The former deputy first minister, who was one of the key architects of the 1998 Good Friday Agreement, died on Friday.
He was aged 83.
Past and present government ministers are among the mourners in St James’ Church in Mullaghbrack, Co Armagh for his Requiem Mass at midday today, Monday, January 27.
Derry-born Archbishop Eamon Martin said he was a “shining example of someone who gives their life in a vocation of service”.
The North of Ireland’s First and Deputy First Ministers Arlene Foster and Michelle O’Neill, Taoiseach Leo Varadkar and British Secretary of State Julian Smith are in the congregation.
Other mourners include former Taoisi Bertie Ahern and Enda Kenny and former First Minister Lord Trimble.
Archbishop Martin, leader of the Catholic Church in Ireland, told those gathered in the tiny church of St James’ that the politician was determined to “make a real difference, and to leave the world a better place than it was when he entered it”.
He said having lived through “the worst of the Troubles” in the North of Ireland, Mr Mallon “personally played a central role in the landmark events of our peace process”.
The archbishop described him as a peacemaker, a bridge builder, a leader and a statesman.
But he added “whether he was with presidents, prime ministers, party colleagues, or his own good neighbours and friends here in Markethill, he was the same Seamus”.
St James’ Church only seats 55 people so the church hall and an additional marquee have been opened to house the overspill.
His coffin was carried into the church by his SDLP colleagues, including party leader Colum Eastwood and former leader Mark Durkan.
During the ceremony, gifts representing Mr Mallon’s life and career were brought to the altar.
They included a copy of the former MP’s maiden speech to the House of Commons and a photo of him with Pope John Paul II.
Other personal items included fishing reel, a pot of roses and a set of golf balls.
Mr Mallon will be buried in the cemetery in the grounds of St James’ church, where he was baptised.
Seamus Mallon’s parish priest, Fr Michael Woods, expressed sympathy for Mr Mallon’s family, including his daughter Orla.“We are deeply conscious that for you, Orla, and the entire family this is an occasion of great personal sadness as you say farewell not so much to a man of high stature in public life on the island, but rather your own dear father,” he said.
He also welcomed all the politicians and public representatives in attendance, saying: “Your presence in such huge numbers is testament in itself to the impact of Seamus on the life of this island.”
The Assembly sat briefly at midday and was suspended, before reconvening at 4.30 om for members to pay tribute to Mr Mallon, before a procession takes place to open a book of condolence.
A separate book of condolence opened at Belfast City Hall on Saturday, while another was opened at the Guildhall in Derry on Sunday.
Mr Mallon was the first person in Northern Ireland to hold the post of deputy first minister, when the role was created in 1998.
At that time, Ulster Unionist leader David Trimble was first minister and although they were dubbed a political “odd couple”, the pair were united against violence.
Mr Mallon served as the deputy leader of the SDLP when Mr Hume was leader of the party.
Both men are widely regarded as playing a key role in the forging of the 1998 Good Friday peace agreement, which established power-sharing government in Northern Ireland.In his long career in politics, Mr Mallon also served as a member of Seanad Éireann.
His wife Gertrude passed away in October 2016 after a long illness.
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