The three countries agreed a deal in principle to govern the trillion dollars of mutual trade after a year and a half of contentious talks concluded with a late night bargain just an hour before a deadline on September 30. "And Donald, it's all the more reason why we need to keep working to remove the tariffs on steel and aluminum between our countries".
Trump praised the deal in the lavish language he uses when touting one of his administration's accomplishments.
The deal is expected to be signed at 7 a.m. ET.
Trump had repeatedly criticized NAFTA, which has governed North American trade since 1994, calling it "the worst trade deal in history".
Top U.S. officials were on hand for the signing ceremony, including Secretary of State Mike Pompeo, Treasury Secretary Steven Mnuchin, national security adviser John Bolton and the President's daughter Ivanka Trump and son-in-law Jared Kushner.
The new agreement has received a mixed reaction from USA lawmakers.
Trump, however, didn't appear to think he'd have a problem getting it through Congress.
"This has been a battle and battles sometimes make great friendships so it's really terrific", Trump said. The longstanding trilateral deal was roundly criticized by the United States protectionist right, as well as the anti-globalization left, particularly for the damage that eliminating tariffs did to the U.S. auto industry.
Friday's signing ceremony is a crowning moment for Trump, who loves to tout his deal-making prowess. -This follows the President's announcement in October that a deal had been reached.
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And even as Trump, Trudeau and Peña Nieto - on his final day in office -signed the accord on the sidelines of the summit, officials from the three countries continued to haggle over terms for lifting USA tariffs, according to The Washington Post.
The signing event and the leaders' remarks are being livestreamed.
Mr Trump tweeted he was axing the meeting with Russia's leader shortly after his former lawyer Michael Cohen admitted lying to Congress about a Trump property deal in Russian Federation.
There are a few areas still to iron out. 'The fact that this is an agreement in three languages adds to the level of technical complexity and it is on that level that we are just being sure that all the Is are dotted and all the Ts are crossed'.
Trump, who arrived in Buenos Aires late Thursday, barreled into the two-day meeting by announcing via Twitter that he was canceling a planned meeting with Russian President Vladimir Putin over Russia's seizure of Ukrainian vessels.
This may prove most challenging for Mr Trump, as the Democrats take control of the House of Representatives from January.
American dairy and pork farmers have been particularly hurt by retaliatory Mexican tariffs.
Legislators from the three countries still have to approve the pact, officially known as the United States-Mexico-Canada Agreement (USMCA), before it goes into effect and replaces the North American Free Trade Agreement (NAFTA).
Trump's Friday schedule also includes an informal meeting with Australia's new prime minister.