Born in Sudbury, Ontario, Desmarais took control of his family’s bus service, Sudbury Bus Lines, in 1951. He then acquired additional bus lines in the Ottawa area and Quebec City (including Quebec Autobus and Provincial Transport). By 1968, Desmarais's holding company, Trans-Canada Corporation Fund (TCCF), owned the bus line Provincial Transport, an interest in Toronto-based Imperial Life Assurance and Gesca Ltée, (which had an interest in the Montreal paper La Presse). That year TCCF made a share-exchange offer with Power Corporation of Canada, headquartered in Montreal, Quebec, whereby Paul Desmarais became Chairman and Chief Executive Officer.
Amongst its numerous worldwide holdings, Desmarais'; Power Corporation of Canada owns 25% of Groupe Bruxelles Lambert, one of Belgium’s top ten companies which, in 2001 acquired control of the German publishing giant, Bertelsmann AG.
In 1974, Desmarais named employee Paul Martin, Jr as president of a Power Corporation subsidiary, Canada Steamship Lines Inc. In 1981, he sold the company to Laurence Pathy and Paul Martin, Jr., the latter who became Prime Minister of Canada on December 12 2003.
Desmarais has two sons: Paul Jr. and André (who is married to former Canadian Prime Minister Jean Chrétien's daughter France).
Paul Desmarais, Jr. is the eldest son of Paul Desmarais, Sr. He is the Chairman and Co-Chief Executive Officer of Power Corporation of Canada.