Paul Desmarais Sr., the Canadian billionaire who built one of the country’s biggest fortunes after inheriting a fleet of buses in a mining town, died Oct. 8 at his estate in Quebec’s Charlevoix region. He was 86.

The death was announced by Power Corp. of Canada, his Montreal-based holding company, which also has investments in Europe and the United States. Mr. Desmarais was the fourth-richest Canadian, with a net worth of about $5.1 billion, according to the Bloomberg Billionaires Index.

The cause of death was not disclosed, but he had suffered a stroke in 2005, the Toronto Star reported.

Power Corp., which had revenue last year of $32.9 billion (Canadian), controls Great-West Lifeco, the country’s second-largest insurer; IGM Financial, Canada’s largest publicly traded mutual fund company; and Swiss holding company Pargesa Holding. Mr. Desmarais, who acquired control of Power Corp. in 1968, served as the company’s chairman and chief executive until 1996, when his sons Paul Jr. and Andre became co-chief executives.

Mr. Desmarais’s position at the apex of Quebec’s business establishment tied him to the halls of power in Ottawa. According to journalist Peter C. Newman’s “The Canadian Establishment,” his close friends have included former prime ministers Pierre Elliott Trudeau, who died in 2000, and Jean Chretien. His younger son Andre is married to Chretien’s daughter, France.

Four former prime ministers have worked for Power Corp., according to data compiled by Bloomberg.

Paul Martin, who spent 15 years working for Mr. Desmarais before switching to politics and becoming prime minister, said his former boss “demonstrated that a Canadian can build a great company that can have a global imprint.”

Mr. Desmarais was also a philanthropist and a patron of the arts. He supported organizations such as Montreal’s Canadian Centre for Architecture, home of the Paul Desmarais Theatre. A pavilion of the Montreal Museum of Fine Arts is named after Jean-Noel Desmarais, his father.

Mr. Desmarais supported a unified Canada, which didn’t always endear him to Quebec separatists such as Marois. In 1970, the Front de Liberation du Quebec, a nationalist group, named Mr. Desmarais as a target in a manifesto. He was then provided with round-the-clock security.

In his home province, Mr. Desmarais was also known for his 15,000-acre Sagard estate in the Charlevoix region, about 300 miles north of Montreal. The property covers an area larger than Manhattan. In a 2008 interview, Mr. Desmarais told France’s Le Point magazine that his chateau was inspired by the Malcontenta de Palladio villa near Venice.

Guests at Sagard have included former French President Nicolas Sarkozy and former U.S. president George H.W. Bush. Mr. Desmarais was awarded the Grand-Croix of the French Legion of Honor, the country’s highest distinction, by Sarkozy in 2008.

Paul Guy Desmarais was born on Jan. 4, 1927, to Jean-Noel and Lebea Desmarais in a French-speaking enclave of Sudbury, Ontario, a nickel-mining town 250 miles north of Toronto.

He began his corporate career in 1951, taking a break in his legal studies to run a family-owned transportation company that his grandfather started in 1916. At the time Mr. Desmarais took over the company, it had 16 buses and $384,000 (Canadian) in debt, according to “The Canadian Establishment.”

Through a series of transportation company takeovers, Mr. Desmarais built a corporation that would go on to acquire newspaper, radio and financial-services properties. He started his foray into financial services by buying control of insurer Imperial Life for about $12 million (Canadian) in 1963. He then acquired the Montreal newspaper La Presse, and by 1968, he had gained control of Power Corp., at one time an electric utility, according to “Billionaires of Canada” by Timothy Le Riche.

Over the years, Mr. Desmarais built a web of European holdings with Albert Frere, a Belgian industrialist who is one of Europe’s richest men. Through Pargesa, the Frere and Desmarais families own stakes in companies such as BNP Paribas, France’s biggest bank, and Total SA, France’s largest oil producer.

In the United States, Power’s Great-West insurance unit completed its biggest acquisition in 2007 with the $3.9 billion purchase of Putnam Investments, a Boston-based mutual-fund manager.

In 1978, Mr. Desmarais was admitted to the Order of Canada, the country’s highest civilian honor. Survivors include his wife, the former Jacqueline Maranger, and four children.

— Bloomberg News