Crowds who gathered at a London park to watch the Games on live screens and enjoy a fun fair were delighted Wednesday when the mayor provided a comic sideshow.
Johnson was trying out a zip line that was 45 metres high and 320 metres long at east London's Victoria Park. It was an unlucky first time: The portly mayor lost momentum about three-quarters of the way across, leaving him dangling from his harness about 10 metres above ground for a few minutes.
"Can you get me a rope? Get me a rope, okay?" he was heard shouting good-humoredly in a video posted to the ITV News website.
The crowds, happily taking photos with their cameras, responded with loud laughter.
Photographs of the mayor — dressed in a dark suit, a blue helmet and clutching two Union Jack flags while dangling on the wire with his socks showing — quickly spread on online social media networks like Twitter.
Staff at the park eventually pulled him to safety with a rope.
Johnson said he probably got stuck because one of the brakes was left on.
"Anyway it was wonderful, I thoroughly recommend it," he told ITV.
A spokeswoman at the mayor's office chuckled about Wednesday's mishap and said Johnson was doing just fine.
"Fortunately, the mayor survived his first zip wire experience," a statement from his office said.
"Clearly the judges are likely to mark him down for artistic interpretation, and unlike Team GB, he won't be bagging any gold medals today but he remains unbowed," it added.
Prime Minister David Cameron acknowledged the political magic that appears to surround the eccentric but popular mayor.
"If any other politician anywhere in the world was stuck on a zip wire it would be a disaster. For Boris, it's an absolute triumph," he said.