The London mayor, who recently invited Rupert Murdoch to be his personal guest for the London 2012 Olympic swimming finals this Friday, became suspended on the mechanism.
It was not clear if Johnson was stuck by mistake or as an intentional publicity stunt.
Johnson was stuck 20 metres from the end of the 320-metre line. He was rescued after being stranded mid-air for five minutes.
"Can you get me a rope?" Johnson asked. "Get me a rope, okay? I think they need to test this on somebody going a bit faster."
The crowd responded by laughing.