O'Donnell's parents, Etta and Henry, were both heavily involved in the local racing circuit. After graduating from high school, he became second trainer for Jim Doherty, and moved to Saratoga, New York, where in 1979 he set the all-time record for most wins (269) at a single track in a single season. Later he went to the Meadowlands in New Jersey on the Grand Circuit.
O'Donnell was given the nickname "Magic" for being able to drive a horse seemingly past its limitations. He was named Harness Tracks of America's Driver of the Year in 1984, a year when he set a yearly earnings record of over $9 million. In a race in 1984 with Nihilator, O'Donnell was the first to post a sub-1:50 race mile. The next year, he became harness racing's first driver to eclipse $10 million in earnings in a single year. He returned to Canada in 2001.
He has been inducted into Canada's Sports Hall of Fame (1992), the Nova Scotia Sports Hall of Fame, the Canadian Horse Racing Hall of Fame (1986), and the Harness Racing Hall of Fame in Goshen, NY (1991). He lives in Acton with his wife Barbara and his 3 children, Christopher, Megan, and Sean.