Speaking at the Group of 20 meeting in Buenos Aires, Argentina, with Trudeau and Peña Nieto on either side of him, Trump acknowledged that the road to a deal had been hard.
All three leaders are in Argentina to attend the G20 summit.
Trump faces a series of diplomatic challenges over the weekend, most notably whether he can strike an agreement with Xi.
Negotiations to lift the tariffs are now underway and Mexican officials are reportedly considering limiting metal shipments to the U.S.to 80 percent of current levels.
The United States-Mexico-Canada Agreement (USMCA) opens 3.6 per cent of Canada's dairy market to tariff-free imports and limits Canada's global dairy exports of skim-milk powder, baby formula and milk-protein concentrates.
Trump imposed the duties earlier this year to protect USA producers, claiming it is in the United States national security to protect those domestic industries.
"So we're hoping that when we see this come all together that not only will we secure the market that we have, but we have the opportunity to grow in Mexico and also resolve some of our differences in Canada", said Trotter.
Ending the NAFTA would have meant tearing up the continent's closely integrated auto supply chain.
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 that 40 per cent of vehicle parts be made by workers paid at least $16 an hour. Democratic Senator Elizabeth Warren said she would vote against the pact unless it is reopened to toughen labour and environmental standards, and does more to stop USA companies from moving to Mexico.
Mexico also agreed to continue to recognize USA auto safety standards, unless Mexican regulators conclude they are inferior to their own standards.
Trudeau's readout on the same call also mentioned lifting the tariffs the USA imposed on steel and aluminum.
And the Canadian prime minister hit Trump on one of the most strained features of the countries' relationship: Trump's tariffs. The United States, for its part, wants Canada and Mexico to accept quotas that would force them to export less of the metals to the United States, said sources briefed on the negotiations.
Mexican negotiators reportedly desire modifications to limit future metal shipments to the United States to 80 percent of current levels, while Canada is resisting quotas in the face of affordable steel from China entering the American market. Both chambers would need to sign off on the agreement before it could make its way to Trump's desk to be signed into USA law.
With an early tweet, Trump defended the continued negotiations by his private company over a proposed luxury tower in Moscow while he was running for president in 2016, saying it was "very legal & very cool".
Mr Trudeau said the deal was a major step for Canada's economy but warned that there is more work to be done.
When NAFTA took effect in 1994, e-commerce hardly existed in its current form and modernizing its provisions was a key premise of the talks.
Automakers are reacting to the news, and a good portion of it is positive.
New rules ensure that data can be transferred cross-border and that limits on where data can be stored are minimized.
Tucked in the agreement is a provision that appears created to stop either Canada or Mexico from seeking a better deal with Beijing. The vast majority of the pact still needs to be ratified by lawmakers in the three countries but the signing enacts a handful of immediate protections, such as from auto tariffs.