In the 1970s, Schwartz worked at Bear Stearns. In 1976, his mentor Jerome Kohlberg, Jr. left, followed by Henry Kravis and George Roberts. The trio later founded Kohlberg Kravis Roberts. Schwartz left Bear Stearns shortly afterward and returned to Canada. Along with Izzy Asper, Schwartz co-founded CanWest Global Communications in 1977.
In 1982, Schwartz married Heather Reisman, Chief Executive of Indigo Books and Music. Schwartz has two children from his first marriage and two stepchildren from his marriage to Heather Reisman. The couple are members of the Reform synagogue, Holy Blossom Temple in Toronto. Schwartz and Reisman own the most expensive house in Toronto, valued at $28 million.
In 1983, Schwartz founded Onex Corporation. Serving as its CEO and majority shareholder, Schwartz grew the company into one of the largest in Canada.
In 2005, he and Reisman started a scholarship program for IDF lone Soldiers called the Heseg Foundation. In 2006, he made a donation to University of Waterloo for an exchange program between University of Waterloo and the University of Haifa.
In 2013, Schwartz and Reisman donated $15 million to Mount Sinai Hospital in Toronto.