That initial event has evolved into a global community of weekly timed 5km parkruns on Saturdays and 2km junior parkruns on Sundays, with more than 2,800 event locations in 23 countries.
Runners held cards to form a “thanks a million” sign at Bushy Park on Saturday.
They were joined by Dame Kelly Holmes, who won 800m and 1500m gold medals at the 2004 Olympics, to celebrate the one millionth event.
The landmark parkrun saw a large turnout, with runners queuing in finish funnels to get their times recorded on a sunny morning in the capital.
Sinton-Hewitt said: “All these people in the queue waiting to be scanned are having a chat with one another, they’re becoming friends and next week they’ll have a little bit of a rivalry with each other, it’s all part of the magic. And really anyone can take part.”