Around half a million signed the document back in 1912 in protest against Home Rule. On Saturday men and women dressed in period costume gathered to re-enact the event.
Inside the City Hall, as 100 years ago, Sir Edward Carson stood before the document which was placed on a table draped with the Union flag, and said: "This is the moment when we pledge ourselves to the defence of Ulster. I will be first to sign."
First Minister Peter Robinson, Ulster Unionist leader Mike Nesbitt and Belfast's Lord Mayor Gavin Robinson were among those who symbolically put their name to the Covenant.
Representatives from Belfast City Council also penned their names, and many others who had gathered at City Hall filed through the doors to add their signatures.
Mr Robinson said: "It was a solemn occasion. The men and women who signed the Covenant and the Declaration were facing into very difficult and dangerous times."
This was something that bound them together to take whatever steps were necessary to defend the Union and here we are, one hundred years later, and the Union is stronger than ever.
The great-grandson of Unionist leader James Craig was in Belfast on Saturday to watch the parades through the city and to sign the Covenant.
"For the family it's an important moment, 100 years on, to revisit and find all the activities that are going on, and get a feeling of how it was back then for our family," explained Max Coleman.
Lord Mayor Gavin Robinson reflected on the significance of signing the Ulster Covenant - both 100 years ago and now, adding that he hoped it would be a "peaceful and enjoyable day".
"One hundred years ago today the Lord Mayor of Belfast, Sir RJ McMordie, stood on this marble with the leaders of Unionism and affirmed themselves to ensure a rightful and a just future for this province of ours.
"I hope we reignite that flame of passion that people had 100 years ago for this place," he said.
Outside City Hall, PUP leader Billy Hutchinson stood in front of the crowds that filled Royal Avenue and read the Ulster Covenant, in which people vowed they would not recognise the legitimacy of a Home Rule Parliament.
It is estimated 30,000 marchers - including 100 bands - then took part in the main parade from the City Hall to the grounds at Stormont to celebrate the centenary.
Ulster Unionist peer Lord Laird explained his family had taken part in the original event.
"All my family at the time of the Covenant, they all signed the Covenant without exception," he said.
"This is making our stand for liberal values, for equality, for the sense of identity."
Historical displays and a cultural village were among the attractions on display at Stormont, where amusement rides and a children's play area were also set up. People of all ages gathered in the grounds, including visitors from England and Scotland.
We must maintain the steadfastness and loyalty of our forefathers as we look forward to the next one hundred years of Northern Ireland within the United Kingdom.
Grand Master Edward Stevenson
Some of those attending the event at Stormont recalled the 1962 commemorations, marking 50 years since the Covenant was signed.
Former Ulster Unionist MLA Ken Robinson told UTV both of his grandparents signed the Covenant.
"It's right and centre of the family history. It's actually very emotional," he said.
"To see their signatures and be able to show my grandchildren my grandfather's signature is fantastic.
"I was here for the 1962 parade and I never thought I would see this one, but it's great to be here."
Orange Lodge chaplain Mervyn Gibson said the day had surpassed all expectations.
"We've had a great day as we always intended we would," he said.
Monaghan Grandmaster Robert Sturgeon came to the city with his lodge to celebrate those from the southern counties who travelled by train to Belfast to sign the Covenant.
He said they had enjoyed the walk from the city centre to Stormont.
"Every mile we did the crowds were applauding and it's something that we are not really used to. With a good band and the crowds applauding you were at Stormont before you knew."