Tony Blair said the Good Friday Agreement came together only “moments” before they announced it to the world.
The former prime minister described the historic 1998 negotiation as a “rollercoaster” that ended decades of violence.
Blair admitted there were some things they could have done differently during the intense 58 hours of talks.
Monday marked 25 years since the Good Friday Agreement was signed in Northern Ireland on April 10, 1998.
The historic moment effectively brought an end to decades of conflict known as The Troubles.
The knife-edge negotiations were led by Blair, then Taoiseach Bertie Ahern and US senator George Mitchell, with Lord Trimble representing the unionists.
Blair told the BBC’s Radio 4 Today programme: “It was an exhausting three days or more and the most intense negotiation I’ve ever been involved in.
“My deepest recollection was just the rollercoaster of that because sometimes it looked like the deal was on and sometimes it looked like it was off.
“It only really came together literally in the moments before we announced it.
“It was an extraordinary time. As the negotiation went on and the focus from - not just the UK media - but the global media grew larger there was this overwhelming sense of desire to succeed because had we failed it would have been a very humiliating and public failure.”
Asked if there was anything he wished he had done differently, Blair added: “Well, there are things within the negotiation that you kind of miscalculate so probably some of the things, yeah, would have been better to have done differently.”
Blair said there were “perfectly reasonable” points to be made about the system of government not being ideal, the time it took to decommission IRA weapons and the way they released prisoners.
“In order for a negotiation like that to succeed, you’ve got to be prepared to have all the different elements and somehow bring them together in cohesion,” he aded.
“But they’re not brought together with all of them being resolved in precisely the way you wanted but the question is does the whole hang together and work?Which fortunately, in this case, it did.”
Blair said the agreement should be “kept under review” all the time but added: “When you think of how it used to be in the 70s, 80s and 90s. You’d wake up every morning to acts of terrorism, death, misery, it was a very difficult time.
“It cast a shadow over the whole of the country. It’s impossible almost now 25 years on to understand exactly what it was like. Belfast was a city that you walked around and there were soldiers patrolling, there was barbed wire everywhere. And if you think of Belfast today it’s a thriving European city.
“I think the economy of Northern Ireland’s doubled in size in the last 25 years so that there’s a lot of positives.”
It comes as Joe Biden is expected to travel to Northern Ireland on Tuesday to meet with Stormont’s main political parties.
The US president, who has Irish ancestry, said will underscore his nation’s “commitment to preserving peace and encouraging prosperity” in NI.
“25 years ago, Northern Ireland’s leaders chose peace,” Biden tweeted.
“The Belfast/Good Friday Agreement ended decades of violence and brought stability.
“I look forward to marking the anniversary in Belfast, underscoring the U.S. commitment to preserving peace and encouraging prosperity.”
Biden will also travel to the Republic of Ireland, where he will carry out a number of engagements during his four-day stay, including visiting Co Louth and Co Mayo, from where his ancestors hail.
Prime minister Rishi Sunak will meet Biden when Air Force One touches down in NI on Tuesday.
The two leaders will hold a bilateral meeting on Wednesday before Biden gives an address at Ulster University.
Stormont’s powersharing assembly - established in the peace deal - is not currently operating due to a protest over post-Brexit trading arrangements by the DUP, the largest unionist party in NI.