Millar's win came a day after Canada's show jumping team of Ben Asselin, Tiffany Foster, Eric Lamaze and Millar claimed the $300,000 Nations' Cup for Canada's second win in the event's 38-year history.
On Sunday, Millar stepped into the spotlight solo. Riding Dixson, owned by Susan Grange and her daughter, Aerial, Millar produced two faultless rounds over the tracks.
Millar, 67, forced a jump-off with 20-year-old Reed Kessler of the United States after a second clear round. Kessler, riding her 2012 Olympic mount Cylana, jumped clear in a time of 49.50 seconds before Millar guided Dixson to a clear round in 47.33 seconds to take the win.
"I can honestly tell you that I never thought I would win this grand prix again," said Millar, who claimed victory in 1987 and 1991. "It was a long time ago, and the sport has changed so much. I'm a little more mature than I was, and these young people go plenty fast and they know all the tricks.
"They are really, really good. I have the greatest admiration for my fellow riders. To go up against them is no easy task."
Millar is ranked third on the all-time money won list at Spruce Meadows behind Eric Lamaze and Beezie Madden, with career earnings totalling more than $3.5 million.
"The afternoon just seems to fly by," Millar said. "It's like a blur, it happens so fast. The tension is incredible. To me, it is the most prestigious grand prix in the world. It's a heck of a payday. It's just a thrill. It is beyond words."