Johnson started out strong, proudly waving a Union Jack in each hand and beaming as he slid down the wire. But as he got to the middle of the wire he got stuck.
He was left dangling halfway down the 320m-long wire in Victoria Park, east London, calling out for a rope or a ladder.
"I think we need to test this on somebody going a bit faster," the mayor joked, as people on the ground scrambled to get him down.
Pictures of the mayor's mishap were soon trending on Twitter as onlookers wasted no time documenting his misfortune.
Johnson was in the park doing a demonstration to garner more support for free Games events from the locals.