Jonathon is the son of Canada's most frequent Olympian and most decorated equestrian, Ian Millar. In 2007 Jonathon made sports history when he finished second to his legendary father, by mere fractions, in the Canadian Championship. They became the first members of the same family to finish first and second in the national show jumping championship. That 1-2 finish was just one in a series of such family finishes that year. It was one of four 1-2 finishes that Jonathon had been part of, with his father or sister, that season.

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"Team Millar" has performed some other family feats over the years, including a 1-2-3 finish in a $100,000 grand prix in Ocala, Florida in 2001. In a three-way, all-Millar jump-off, Jonathon won, followed by his father and then his sister. It was Jonathon's first grand prix win. In 2000, all three Millars were members of the team that represented Canada in the nations cup at the prestigious Spruce Meadows Masters tournament, in Calgary. 

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This year, Jonathon was a member of the Canadian team for a short European tour, during which he contributed a clean round to a second-place team finish in the nations cup in Barcelona.

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Jonathon first tasted international success at the North American Young Riders Championships (for riders 16 to 21 years of age). He helped the Ontario team to gold and silver medals in 1992 and 1993, respectively. Just one year later, in 1994, Jonathon made his Canadian Equestrian Team debut, in Washington D.C. 

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From 1997 to 2000, Jonathon was based in Europe, working and training with Dutch team veteran Emile Hendrix. Returning to Canada in time to contest the Olympic selection trials, he was named second alternate to that team, moving up to first alternate before the Games began. 

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Standing 6'4", Jonathon was an avid basketball player throughout high school. Along with his sister and father, this member of "Team Millar" is an integral part of the training, coaching and horse sales business at Millar Brooke Farm.