The Ontario-born businessman remained a staunch Canadian federalist after he moved to Quebec, where he became one of the country's wealthiest and most powerful figures whose influence straddled the worlds of business, politics and philanthropy.
His family said he died peacefully Tuesday evening at his country estate northeast of Quebec City surrounded by relatives, including his wife and four children.
His Power Corp. was not only a conglomerate. It was a pipeline into the Prime Minister's Office.
Even when Pierre Trudeau, Brian Mulroney, Lucien Bouchard, Jean Chretien and Paul Martin weren't speaking to each other, they shared a common friend in Desmarais. In Chretien's case, they even became family.
Martin, who launched his business career at Power Corp., said his old boss played a key role in the 1995 Quebec referendum campaign, telling other business leaders to speak up in defence of Canada.
Although Desmarais riled many separatists, Martin said he was a great example during the 1960s Quiet Revolution, proving that French-Canadians could succeed in business.
"For Quebecers, for Acadians, for Franco-Ontarians and for French-Canadians right across the country to succeed ... Paul Desmarais was the living symbol of that," Martin said in an interview.
"But he did it really with such grace and with such character and integrity — I also think that was part of that example he set."
Former adversaries also offered their respects Wednesday. Parti Quebecois Premier Pauline Marois described him as "a great builder who contributed to advance Quebec's economy."
His influence was exercised far beyond Quebec.
When Desmarais threw a housewarming party in 2003, the guests included two former U.S. presidents — Bill Clinton and George H.W. Bush.
Nicolas Sarkozy, meanwhile, once credited Desmarais with helping make him the president of France. Sarkozy returned the favour in office, becoming the most ardent critic of Quebec independence among all the recent occupants of Paris' Elysee Palace.
His patronage of the arts included contributing to the restoration of the famous gardened estate of impressionist painter Claude Monet.
Buildings at universities in Montreal and Ottawa carry his name.
"Mr. Desmarais will be remembered as a unique business leader who improved the lives of Canadians through the creation of jobs and through charitable endeavours," Prime Minister Stephen Harper said in a statement.
The path to power began in Sudbury, Ont., where he was born in 1927. He left law school to take over the family's ailing bus company in 1951.
A series of smart moves resulted in the creation of a holding company that in 1968 made a share-exchange offer with Power Corp.
With his company's diversified holdings in insurance, transportation, paper, media, and financial services, Desmarais was one of the most notable members of his province's business elite, often referred to as Quebec Inc.
His empire included Great West Life, London Life and Canada Life in the insurance industry; the Investors Group and Putnam Investments; the Gesca newspaper chain, with its flagship Montreal La Presse; and stakes in the oil company Total S.A. and the Pernod Ricard liquor company.
He was discreet when it came to discussing politics.
As a media baron, his style was the antithesis of a Rupert Murdoch who regularly uses a social-media platform to opine on the political story du jour.
Only on rare occasions did he voice his views publicly. During his last annual meeting as Power's chief executive in 1996, he extolled his belief in Canadian unity.
"My profound attachment to Canada stems from the great liberty and freedom that my ancestors were able to enjoy in building their lives in a new country, the same liberty and freedom which allowed me as a young French-Canadian from Northern Ontario to realize his dream in building a business in all parts of Canada and abroad."
In interviews Wednesday, Chretien and Mulroney said he would offer his opinion when asked — but never tried using his infuence to change government policy.
"A lot of people listened when Paul Desmarais said something, so of course he was powerful, but he was powerful in the open," Chretien said from Rome.
The former Liberal leader, who shared four grandchildren with Desmarais through Chretien's daughter France, recalled their many fishing trips together and Desmarais' companionship and love of jokes.
He said the family patriarch was in a good mood when they last met a few weeks ago even though he had been in poor health and knew death was near.
"He was living in his house in Charlevoix since many, many months and he told me that he was expecting to die there."
Chretien described Desmarais as a great success who became a role model for many French-Canadians by demonstrating that being a minority in Canada is not a handicap.
Mulroney said his friend of 48 years was a "renaissance man" who loved art, books, architecture and opera but wasn't interested in being a political kingmaker, even though he was willing to "spread the word" for people he liked.
"And, believe me, that couldn't do anybody any harm," said the former Conservative leader, who saw Desmarais just last weekend.
Mulroney said Desmarais admired politicians of all parties because of the sacrifices they make: "I know that he was an admirer of Rene Levesque because Rene Levesque was a real democrat."
That might shock some of the nationalist critics who accused Desmarais of using his editorial pages to combat Quebec independence.
They also accused him of persuading his friend, Sarkozy, to adopt a pro-Canada foreign policy.
Sarkozy himself admitted he owed much to Desmarais.
"If I am the president of France today, it is thanks in part to the advice, the friendship and the loyalty of Paul Desmarais," Sarkozy said during a 2008 ceremony awarding Desmarais the Grand Cross of the Legion of Honour, France's highest order of merit.
One La Presse columnist got so fed up with nationalist conspiracy theories involving his boss that he wrote an entire column on the subject last year.
Pierre Foglia described himself as a committed, left-wing separatist who had not heard a single complaint in 45 years of working for a less left-wing, non-separatist boss.
Foglia said he'd received his first phone call from the boss only a year earlier. They'd never spoken before. He said Desmarais only called because he wanted to help someone he'd read about in a column. And he wanted the deed kept quiet. He said Desmarais called again a couple of weeks later, to borrow some books.
That didn't stop some separatists from casting Desmarais in bogeyman terms, as some puppet-master pulling politicians' strings from the shadows.
Following news of his death, one pro-independence hardliner who sits on the executive of the small Option nationale party, Patrick Bourgeois, accused Desmarais of spreading capitalist and federalist propaganda in a scornful eulogy titled, "The worms will finally eat the good Mister Paul."
Many others, however, were singing his praises Wednesday.
One Quebec politician, Francois Legault, said he looked up to Desmarais as a role model before he launched his own business career in the airline industry. He said he was inspired by Desmarais' biography, which he read when he was 20.
Some said Desmarais had the golden touch — that he knew when to sell assets.
Asked about the key to his business success, Desmarais simply responded: "I don't know. I seized opportunities that presented themselves."
For example, concerns about the future of the pulp and paper industry prompted Power to sell Consolidated-Bathurst for $1.5 billion and net more than $1 billion in the process.
Power Financial sold its shares of Montreal Trust to Bell Canada parent BCE Inc. for $547 million in 1989 because of reservations about the ability of a mid-sized company to compete with Canada's large chartered banks.
He helped open the door to Canadian businesses in China by leading a commercial delegation there in 1978. He was the founding chairman of the Canada China Business Council.
However, it took eight years before Power Corp. launched a business venture there. It now invests in infrastructure projects in China through its stake in CITIC Pacific Ltd.
By the time he handed daily operations of the company to his sons in 1996, Desmarais had seen Power's assets increase to $2.7 billion, from $165 million.
Power Corp., through its Square Victoria Communications Group subsidiary, and together with the corporate parent companies of the Toronto Star and Globe and Mail newspapers owns The Canadian Press.
Canadian Business magazine ranked Desmarais as the wealthiest Quebecer and Canada's seventh-wealthiest person, with a fortune estimated at $4.4 billion.
An art lover, Desmarais had one of Canada's largest private art collections. Two wings of Montreal's Fine Arts Museum are named in honour of his family.
Desmarais also used his fortune to build one of the world's most exclusive golf courses on his sprawling 75-square-kilometre Sagard estate in the mountainous Charlevoix region of Quebec.
He suffered what company officials described as a "minor stroke" in 2005.
Desmarais is survived by his wife Jacqueline Maranger, sons Paul Jr. and Andre, daughters Sophie and Louise, as well as 10 grandchildren.
A private family funeral is planned, followed by a memorial service to be announced by the Desmarais family.