IMG said the 72-year-old McCormack had immediate medical attention and was hospitalized. The company did not say where he was being treated.
McCormack was picked as one of the top 10 most powerful people in sports last December by The Sporting News.
His Cleveland-based company represents dozens of major sport stars including golfers Tiger Woods and David Duval, Yankees shortstop Derek Jeter, tennis stars Venus Williams and Jennifer Capriati as well as Cleveland Browns quarterback Tim Couch.
IMG has 85 offices in 33 countries and employs almost 3,000 people.
Bob Kain, president of IMG North and South America, and Alastair Johnston, president of IMG International, will oversee day-to-day operations of the firm.
McCormack is credited with being a pioneer in sports management and marketing. He was a lawyer in Cleveland when he founded IMG in 1960, with golfer Arnold Palmer as his first client.
Soon afterward, he branched into representing tennis players, and then athletes in other sports.
After graduating with a law degree from Yale University, McCormack served in the Army before accepting a position with Arter & Hadden, a Cleveland law firm.
McCormack has two sons and a daughter from his first marriage, all of whom work for IMG.
In 1986, he married tennis champion Betsy Nagelsen, who gave birth to another daughter.
McCormack is the author of five books on business and management.
In 2001 the PGA of America awarded McCormack its distinguished service award, given to a person who has a universal impact on the sport. Golf Magazine has called McCormack 'the most powerful man in golf.''