Thomson, who owned about 70 percent of Thomson Corp., had become chairman of the global information company after the death of his father, Roy, in 1976. He turned the chairmanship over to his son, David, in 2002.
Thomson remained on the board of Thomson Corp. and headed the Woodbridge Co., his family's private investment company. He was ranked ninth on the Forbes magazine list of the world's wealthiest individuals with an estimated fortune of $19.6 billion.
Thomson died Monday morning at his office in Toronto, a company spokesman said, adding that the cause of death was not immediately known.
Ken Thomson was the third child and only son of Roy Thomson, the hard-driving creator of a business empire that began with a tiny radio station in northern Ontario and grew to embrace Canada's dominant newspaper group and other interests ranging from North Sea oil to travel agencies, retailer the Hudson's Bay Co. and the Times of London.
Under Ken Thomson, the Thomson Corp. and its corps of non-family managers sold most of the company's newspapers and other holdings to concentrate on providing specialist information to legal, investment, medical and other professionals, largely in electronic formats.
Thomson's businesses include Thomson Financial, an electronic provider of financial information, and the legal information company Westlaw.
The corporation, with 40,500 employees and 2005 revenue of US$8.5 billion, now has its operational headquarters in Stamford, Connecticut, although its formal head office remains in Toronto.
At the time of his death, Thomson controlled about 70 percent of Thomson's shares, and that control remains with his family.
Thomson's death is unlikely to have a material impact on the company, said Brendan Caldwell, president of Caldwell Securities Ltd., who recalled encountering the billionaire "in the oddest places."
Those places included a bluegrass concert, a dark and deserted area on the fringes of the upscale Rosedale district where Thomson was walking his dogs, and, most recently, having Sunday brunch at a Golden Griddle family restaurant.
"He was his own man," Caldwell said, lauding him for providing "wisdom and counsel" to Thomson Corp.'s professional managers and "proving that you can hire brilliant executives for low-single-digit millions."
David Thomson lauded his father's "wise stewardship of our family interests for more than 30 years," adding: "More importantly, he was a gentle and kind man who impressed everyone with whom he came in contact. He was much loved."
"He was a remarkable man who did so much to build this business by constantly investing in the future," said Richard Harrington, CEO of Thomson Corp.
"He was a strong leader whose energy and enthusiasm for Thomson was contagious. Anyone who met Ken was touched by his grace, charm and humility."
Ken Thomson served with the Royal Canadian Air Force during World War II, then attended Cambridge University and later joined his father's newspaper business.
Born on Sept. 1, 1923, in Toronto, Ontario, he was a private man who gave an impression of shyness in public, and a major collector of art, focusing on old-style Canadian works by painters like Paul Kane, Cornelius Krieghoff and the Group of Seven.
He recently donated most of his 3,000-piece collection to the Art Gallery of Ontario, along with a substantial gift of money.
Author Peter C. Newman, who interviewed the billionaire for a 1991 book, found Thomson took little pleasure in his position.
"He's not a very happy man," Newman wrote in "Merchant Princes," a book about the Hudson's Bay Co. "He doesn't enjoy life, with the exception of his art."
Thomson wore inexpensive suits and his shoes were occasionally seen to be down at heels, as he seemed determined to play down his wealth and stay out of the public eye.
He inherited the title Lord Thomson of Fleet from his father, but did not use the title in Canada and never took his seat in the British House of Lords, and he rarely entertained at his Toronto mansion.
He is survived by his wife, Marilyn, three children and their families.