Ian Millar of Canada rode into Olympic history on Saturday to set a record before his horse jumped its first obstacle at Greenwich Park.
London marks Millar's 10th Olympics, more than any other athlete in games history.
The 65-year-old Millar 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.