With his horse, Big Ben (1976–1999), Millar won more than 40 Grand Prix titles worldwide and the Show Jumping World Cup two years in a row. At the Pan American Games in August 1987, Ian Millar became the second Canadian to win an individual gold medal. He holds the North American record for Grand Prix and Derby wins. He has been a member of every Canadian Equestrian Team at the Show Jumping World Championships team since 1972. At the 2008 Summer Olympics, Millar competed at his ninth games, tying the record set by Hubert Raudaschl.
On August 18, at the 2008 Summer Olympics in Beijing, at the age of 61, Millar anchored his team (also including Jill Henselwood, Eric Lamaze, and Mac Cone) to a first-place standing. Riding ‘In Style’, he came through in the clutch with a perfect run to lead the Canadian team into a jump-off for gold with the United States. In the team equestrian gold-medal jump-off, Canada won the silver medal, Millar's first Olympic medal.
Millar was named to the 2012 Olympic team, marking his tenth appearance, a record for any Olympic athlete's appearances at Olympics. In the 2012 Olympics' Individual Jumping event, Millar finished in a three-way tie for 9th, the best Canadian result. In Team Jumping, Millar, along with fellow riders Jill Henselwood and Eric Lamaze, scored Canada a fifth place finish. Throughout his career, Millar is a nine-time winner of the Canadian Show Jumping Championship and has captured six Spruce Meadows Derbies. In 1986 he was made a Member of the Order of Canada and in 1996 was inducted into the Canadian Sports Hall of Fame.