Uncertainty would have only worsened and caused additional economic damage had the deal not been reached, Trudeau added.
The official signing took place at the G20 summit in Buenos Aires, after more than a year of negotiations.
USA tariffs on Canadian steel and aluminum will remain in place for the time being, Freeland's office has confirmed.
The signing of the trade pact came to fruitation after more than a year of contentious negotiations that came right down to the wire.
All that changed this week, said one insider: "At the end of the day, removing the uncertainty from the rest of the economy is too important to pass up".
Climate change wiped out siberian unicorns 36000 years ago
Good to know what we have to look forward to! These climactic disruptions proved fatal to many species, Elasmotherium among them. Also, it seems that this "Siberian Unicorn" put the basis of the famous unicorn myth which has been around for millennia.
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. It also imposes a limit on how many dairy products Canada can export and eliminates tariffs on margarine and whey.
The prior belief was that lower-ranked officials would be signing the new trade deal but reporters in Buenos Aires now say that the leaders will be taking part.
The deal is expected to be signed at 7 a.m. ET.
Trump forced Canada and Mexico to renegotiate the 24-year-old agreement because he said the existing pact encouraged USA companies to move jobs to low-wage Mexico.
"Battles sometimes make great friendships, so it is really terrific", Trump said.
Trade will dominate Mr. Trump's conversations at the meeting of the world's leaders, including a high-stakes dinner Saturday with Chinese President Xi Jinping.
China hopes for positive results from talks with US at G20
The G20 meeting comes as global economic growth may be slowing more than expected, the International Monetary Fund warned. Xi said he was "very happy" to talk to Trump again.
Trump acknowledged that Friday was Pena Nieto's last day in office and congratulated him on the achievement of signing on to the deal.
In an interview with CNN earlier this month, Trudeau indicated Canada still wanted the tariffs lifted before the new version of NAFTA took effect, but said the country wasn't at the point where they wouldn't sign the pact if the levies weren't lifted.
USA lawmakers have already indicated they don't expect to tackle the USMCA until after the new Congress is sworn in early next year.
According to the White House, the United States-Mexico-Canada Agreement rebalances the trade relationship with both countries, secures a stronger deal for American industries and modernizes trade practices for a new era.
USA objections to Canada's protected internal market for dairy products was a major challenge facing negotiators during the talks, and Mr Trump repeatedly demanded concessions and accused Canada of hurting U.S. farmers.
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 United States tariffs, according to The Washington Post.
Cowboys-Saints Has Biggest Thursday Night Football Rating In History
I've contended since about five games into the Saints' 10-game winning streak how this team was impervious to a bad performance. On the offensive end, Dak Prescott was the definition of efficient, completing 24-of-28 passes for 248 yards and one touchdown.
"These measures will support many - hundreds of thousands - American jobs", he said.