That move set off retaliatory tariffs and intense negotiations to create a new trade pact.
The signing ceremony took place on the sidelines of the G20 summit in Buenos Aires.
The new agreement doesn't lift the tariffs on steel and aluminum, with both the Mexican and Canadian governments stating their displeasure that the issue has not yet been solved.
He described General Motors' plant closures that had hit thousands of United States and Canadian families as a "heavy blow".
"Donald, it's all the more reason why we need to keep working to remove the tariffs on steel and aluminum between our two countries", Trudeau said.
Trump praised the deal in the lavish language he uses when touting one of his administration's accomplishments.
Mr Trump forced Canada and Mexico to renegotiate the 24-year-old agreement because he said the existing pact encouraged United States companies to move jobs to low-wage Mexico. The White House said USMCA will govern $1.2 trillion in trade, making it the largest trade deal in USA history.
'The new agreement will assure a new prosperity, ' he added. Elizabeth Warren, D-Mass., a likely Democratic presidential candidate, said Thursday that she would oppose the trade pact as inadequate for American workers, foreshadowing a possible 2020 campaign plank.
And the legislative bodies in each nation will have to ratify the treat. The Canadian Conservatives have accused Trudeau of making too many concessions to Trump, who had threatened to "ruin" the Canadian economy unless he got his way. He struck a more optimistic note on Friday, saying, "It's been so well-reviewed, I don't expect to have very much of a problem". "With our signatures today we will formally declare the intention of our three countries to replace NAFTA with the USMCA".
The ceremony is scheduled to start any moment.
Uncertainty would have only worsened and caused additional economic damage had the deal not been reached, Trudeau added.
There is no formal meeting between Trump and Trudeau on either leader's schedule for the Friday and Saturday summit.
There are a few areas still to iron out.
In Mexico, Andrés Manuel López Obrador will also take over the presidency from Nieto, though he indicated his support for the new agreement.
However, after a deal was agreed in principle on 30 September, Mexican President Pena Nieto hailed it as a "win-win-win".
Canadian Prime Minister Justin Trudeau's spokesman only confirmed his attendance late on Thursday. 'The two leaders discussed their disappointment in the announced closures of General Motors plants in their respective countries and their plans for the upcoming G20 Summit'. Canada's dairy lobby urged Trudeau this week not to sign the agreement unless it was changed to give the US less influence over the industry.
The deal - 32 chapters, 11 annexes and 12 side letters - sets new rules for the auto sector, including a higher threshold for North American content and rules requiring 40 per cent of vehicle parts be made by workers paid at least $16 an hour.
Trump's Friday schedule also includes an informal meeting with Australia's new prime minister.
Ohio Sen. Brown: Trump Supports American Cars Act
Trump threatened Tuesday to cut off all federal subsidies to General Motors because of its planned massive cutbacks in the U.S. Steel also said it was upgrading its Gary, Indiana plant. "You better get back in there, '" VOA News quoted Trump as saying .