Prime Minister Justin Trudeau, speaking at the Women in the World summit, was circumspect about the talks, saying only that he remains hopeful.
Trump almost tore up the NAFTA pact past year after visiting farmers in Wisconsin, a major USA dairy producer that Washington says has been hurt by Canadian protectionism.
"There is not going to be a deal - period - unless there is a compromise in that area because that is what he campaigned on and it's stuck to him and he's stuck with it", Mulroney said.
"So if we don't find some way to accommodate some of that, then we're not going to have a deal".
Trudeau replied, "Sorry to disappoint, but we continue to work hard and we are positively optimistic that we can get a win-win-win for all three countries".
Following the talks in Washington, Freeland will travel to Saskatoon, western Canada, on Wednesday to attend a meeting of Liberal members of parliament, which runs until Thursday, Austen said.
"He has great confidence and trust in Lighthizer".
Canadian Foreign Minister Chrystia Freeland sounded sanguine on Wednesday as she emerged from negotiations with top USA trade negotiator Robert Lighthizer, although she cautioned that no trade deal was done until the last issue was nailed down.
True to form, when asked about Mulroney's remarks, the foreign affairs minister refused to say anything publicly about the discussions around the table for fear of undermining what she described as a cordial and productive atmosphere.
A spokesman for Freeland said on Monday that no decisions on whether she would return to Washington later this week had been made.
"We're negotiating all this at once".
NAFTA negotiations resumed Tuesday morning as Americans marked the 17th anniversary of the 9/11 terrorist attacks. "That's important and that's been a success due to Canada and the United States working together and due to Mexico showing some significant flexibility over the summer", Freeland said. "At the end of the day, neighbors help each other when they need help".
But U.S. economists argue the dispute resolution is "on the balance".
Trudeau says Trump's demands are in part linked to talks to update Nafta.
As the two sides met, new economic data showed that the USA trade deficit with Canada grew to $3.1 billion in July.
After a year of effort, Washington and Mexico City announced an agreement in late August and Trump's administration has informed Congress it plans to sign a new treaty by November 30, including Canada if possible.
US officials say time is running out to reach a deal that the current Congress can vote on.
Uncertainly over the future of NAFTA, which underpins $1.2 trillion in trade, is weighing on markets as well as the Canadian and Mexican currencies. President Donald Trump, in a story that appeared in Automotive News Canada, said Friday that while "I don't want to do anything bad to Canada".