London marks Millar's 10th Olympics, more than any other athlete in games history.
The 65-year-old Perth, Ont., native surpassed Austrian sailor Hubert Raudaschi, who retired in 1996 after appearing in nine games.
Millar has been an Olympic competitor for 40 years, his first games being Munich in 1972. The only games he missed in that stretch came during the Western-led boycott of Moscow in 1980.
After his round, in which his horse Star Power knocked down one fence, Millar said he feels the same now physically as he did when he started his Olympic career.
"I am better now than I was then in knowledge and experience," said Millar. "The age of the top riders tends to be older because it takes a lot of time to be consistent."
Millar twice won jumping's World Cup but has won only one Olympic medal, team silver in China in 2008. He contributed a clean round there to bolster his team despite riding with a broken hand.
He hasn't ruled out the possibility of competing in the 2016 Games in Brazil, but his horse's age is more the issue than his own.
"Star Power is 11 now, so the next time around he'll be 14," said Millar. "If he is willing, I am willing."
He said dropping a rail in the preliminary round Saturday was better than in the two-day team competition, which starts Sunday.
"I would have preferred one less rail," said Millar, "but better tomorrow or the next day."
Also on nine Olympics is Latvian shooter Afanasijs Kuzmins, 65, who returned to Olympic competition last week after a hiatus. His first Olympics appearance was in Montreal in 1976, and he won gold in 25-meter rapid-fire pistol in Seoul in 1988.
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