[6] He has been a staple on the Canadian Equestrian Team for decades, and has amassed over $3.5 million[7] in prize earnings at the prestigious Spruce Meadows venue in Calgary, Alberta.

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 (1988 & 1989).

On 18 August, 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.

[9] In the 2012 Olympics' Individual Jumping event, Millar finished in a three-way tie for ninth aboard his gelding Star Power, the best Canadian result.

In Team Jumping, Millar, along with fellow riders Jill Henselwood and Eric Lamaze, scored a fifth-place finish for Canada.

On September 14, 2014, Millar won the $1.5-million CP International at Spruce Meadows aboard Dixson, who shares bloodlines with Big Ben.