President Clinton's role in the peace process will never be forgotten, the British Prime Minister said.
Speaking at the Odyssey Arena before Mr Clinton, Mr Blair told him Britain and the US had always been close. "Our countries have long had a unique relationship and that will always be there," he said.
"Today I want to say thank you not just to the United States but to you personally, your commitment, your intelligence, your encouragement, that Clinton magic that has taken us forward at key moments . . . You may be serving the end of your term in office, but I know and we know that your commitment to peace will never end."
Mr Blair said that since the President's first visit in 1995 there had been "big changes, that we sometimes take for granted: unemployment down by 40 per cent, the fastest growing economy in the whole of the UK here in Northern Ireland. People returning, not leaving, exports doubling, inward investment doubling.
"Without our process of peace these benefits would never have come to the people of Northern Ireland and let us not forget that progress when we talk about the difficulties." Mr Blair said progress would be gradual.
"It was never going to be the case that suddenly one day in April 1998 there were the Troubles and the next day there was peace. It is a process that is painstaking, difficult, full of obstacles."
He said there were still extremists who "hanker after violence and the old ways".
The best answer to those was that "when they engage in violence, let us say to them: `You will not stop the expressed will of the people being done.' "
Mr Blair said the Belfast Agreement still offered `the only way forward" as "the principles at the heart of it are the right principles".
He said to overcome the difficulties political will would not be enough, it would take `the will in each part of the community to implement the agreement we have made".
He said "there are the doers and there are the critics, there are the strivers and there are the cynics.
"There are the ones who get their hands dirty trying and the ones whose hands are clean because they never try."
The Prime Minister said because of their efforts and the risks they had taken Mr Mallon and Mr Trimble would be "the men future generations will salute".
All of those that who were making the effort were doing so because they had one thing in common.
"However much sometimes we disagree and however much sometimes it's difficult, we all know there is no other way forward. There is no alternative.
"There is no other choice that somehow offers a painless way of grasping the potential of the future."