Trump said on Friday that the trade pact was "model agreement that changes the trade landscape forever".
Earlier this month, Yerxa's trade council and 33 other industry groups urged the administration to drop its metals tariffs on shipments from the North American nations, complaining that they are driving up costs and hurting sales.
Perhaps its most significant changes are related to the auto industry, for which Trump insisted on new protectionist provisions meant to wrest some manufacturing back from Mexico and overseas.
The deal comes after more than a year of negotiations between the parties, with agreement on new vehicle and dairy industry regulations proving particularly challenging.
In a letter released Thursday, the Dairy Farmers of Canada warned that the text of the deal gives the USA too much say over how the Canadian system is managed, and urged the government to stand down until that provision is excised.
Mr Pena Nieto, who will be replaced on Saturday with Mexico's newly-elected president, has sold the deal to his people as a victory, saving free trade from a U.S. president who wanted to see it killed.
What's in Trump's new trade deal with Mexico and Canada?
Both Trudeau and Trump have taken criticism over the new agreement.
Mr Trudeau said the deal was a major step for Canada's economy but warned that there is more work to be done. Canada's dairy lobby urged Trudeau this week not to sign the agreement unless it was changed to give the USA less influence over the industry.
Saying all three countries will benefit from the deal, Trump said of the USMCA, "It is probably the largest trade deal ever made".
President Trump, his Mexican counterpart Enrique Pena Nieto and Canadian Prime Minister Justin Trudeau have officially signed the replacement to NAFTA, in the wake of the Trump administration enacting tariffs on steel and aluminum products from Canada and Mexico, which sparked retaliatory tariffs and negotiations with the two countries.
Minister of Foreign Affairs Chrystia Freeland speaks to media on the roof of the Panamericano Hotel in Buenos Aires, Argentina, on November 29, 2018. "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".
It is possible substantive negotiations will resume at some point. The president tweeted in March that the steel and aluminum levies would "only come off if new & fair NAFTA agreement is signed".
Relations between the USA and Canada took repeated blows during the negotiation Trump initiated in August 2017, largely because of his public attacks on Trudeau. But the president, and chief trade negotiator Robert Lighthizer, described them as providing valuable leverage in the dealmaking.
Though the unresolved tariff battle left a cloud over the signing, Friday's scheduled ceremony still represents a signature moment for Trump. "It's great for all of our countries".
Outgoing Mexican President Enrique Pena Nieto is attending the G20 meeting.