In an interview with CTV's "Power Play" to mark this week's anniversary, the former prime minister sternly rebuked Harper for his public spat this year with Beverley McLachlin, chief justice of the Supreme Court of Canada.
"You don't get into a slagging contest with the chief justice of the Supreme Court of Canada, even if you thought that he or she was wrong," Mulroney said in the interview broadcast on Thursday.
"You don't do that."
The spat centred around the failed appointment of Federal Court judge Marc Nadon to the Supreme Court. Harper drew stiff criticism for suggesting McLachlin had acted inappropriately by trying to flag potential problems with the appointment.
Mulroney's criticisms of the government's approach on other issues were equally harsh, particularly on foreign affairs and the Conservatives' tense relations with the United Nations.
"When Canada, for the first time in our history, loses a vote at the United Nations to become a member of the Security Council ... to Portugal, which was on the verge of bankruptcy at the time, you should look in the mirror and say: 'Houston, I think we have a problem.'"
The former prime minister said the government's foreign affairs policy "has to be enveloped in a broader and more generous sweep that takes in Canadian traditions and Canadian history in a much more viable way.
"We're in the big leagues ... so we have to conduct ourselves in that way. We can't be out-riders."
He also chastised Harper for his relationship with U.S. President Barack Obama, suggesting the prime minister doesn't have the kind of close ties that allow him to call the Oval Office and ask for bilateral backup on a number of issues.
"If you can't do that, you don't have much clout internationally. The relationship with the United States is something the prime minister alone has to nurture the same way he would tend to the most delicate flowers in a garden. It's that important."
While the 75-year-old Mulroney said he supported the government and "what they're trying to do generally," he suggested Liberal leader Justin Trudeau is going to benefit from what he believes is widespread public fatigue for the Tories.
"He's a young man, attractive, elected two or three times to the House, attractive wife, beautiful kids -- this is a potent package when you're running in these circumstances," Mulroney said of the eldest son of his one-time political foe.
He disputed those who accuse Trudeau of having no program. "His program is that he's not Stephen Harper."
The same phenomenon was at play, he added, when he was elected due to public antipathy for Pierre Trudeau, while Jean Chretien benefited when Canadians tired of Mulroney.
Mulroney also suggested the Tories are underestimating Trudeau.
"You'd have to be foolish to sit back and not recognize if somebody's leading in the polls 14 months in a row, this is not a fluke," he said in the interview conducted in his Montreal office.
Mulroney also urged the Conservatives to make the environment a top priority, saying a "pristine environment" is important to Canada's middle class.
"There are very few things that the middle class value more than the environment ... and that's one thing we can deliver on," he said.
"The prime minister alone has to make it a very strong priority of the government, has to make sure it has the funds and the clout."
Mulroney also weighed in on the recent debate over whether there should be an inquiry into missing and murdered aboriginal women, saying he would launch such a commission. Harper has rejected widespread calls to set up such an inquiry, suggesting it's a matter for police.
The Prime Minister's Office didn't immediately comment on Mulroney's criticisms.
It wasn't all critiques. Not only did Mulroney repeatedly praise Trudeau, but also NDP Leader Thomas Mulcair, calling him "the best opposition leader since John Diefenbaker."
In September, 1984 Mulroney led the Conservatives to one of the most smashing election wins in Canadian political history.
Note to readers: This is a corrected story. A previous version had an incorrect age for Mulroney
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