The funny incident occurred while he was above Victoria Park wherein giant screens have been erected so that the public can watch events of the ongoing 2012 London Games.
The mayor was supposed to make a grand entry to the Olympic event on the zipwire. At first everything went according to plan.
With the Union Jack in both his hands, Johnson who was wearing a blue helmet and a business suit waved to the crowd even as the announcer tried to build up the excitement by wondering aloud, "Is it a bird? Is it a plane?"
Though Johnson initially gathered speed, soon he got stuck. In an attempt to retrieve the situation with some humour, the mayor sought help from those down below. "Can you get me a rope? Get me a rope, ok? I think we need to test it on somebody going a bit faster," he hollered.
However, it was the officials who came to his aid. They pulled him to safety. When an ITV reporter wanted to know why he was left dangling, Johnson replied, "I think possibly what had happened was that they left a brake on or something. But anyway, it was wonderful and I thoroughly recommend it."
The mayor also told a local radio station LBC that he was "the Yuri Gagarin of the zipwire". Johnson was referring to the Soviet cosmonaut who became the first person to go to space in 1961.
"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," the mayor's spokesperson later quipped about the impromptu gymnastics routine staged by her boss.
The British Prime Minister's remark was perhaps the best. "If any other politician anywhere in the world was stuck on a zipwire, it would be a disaster. For Boris, it's an absolute triumph," David Cameron said.
Incidentally, this is the second time that the mayor has ended up in an unexpected place. In 2009, Johnson fell into a river during a volunteering campaign.