Cycling-mad Rob Holden and pals Ian Laurie and Matthew Winstone hired one of the street bikes in London and put it in a van and then drove onto the Eurostar train.
They made their way 800 miles to Mont Ventoux in the south of France where Rob reached the top of the 1,912-metre peak using just one gear.
The trio then drove all the way back to London and managed to return the bike just 22 seconds short of the 24-hour deadline which would have incurred a £150 fine.
Rob said: “We went for a cycle and stopped for a coffee.
“Out of the blue, Ian just said ‘I wonder if you can rent a Boris Bike and take it down to Mont Ventoux, ride it up and bring it back within 24 hours.’
“We all had a bit of a laugh at that, but then we decided to give it a go.”
Rob paid £2 to hire the heavy 22kg bike at 3:59 on Saturday October 26 from a docking station in Southwark, London.
The trio then bundled it into a van, and drove to the Eurostar, and then made their way more than 720 miles across France to the base of the 6,270ft mountain.
Supported by his pals, Rob battled against cramp, exhaustion and the bike to pedal the 14-mile route to the top of the mountain in an impressive 2 hours and 55 minutes.
He took the toughest southern route which has an average gradient of 7.43 per cent.
Rob battled 25 degree heat, cramp and a heavy bike, unlike professional riders who have 20-plus gears and ultra-lightweight frames.
Rob said: “I had no idea what to expect. I was just taking it one kilometre at a time.”
After reaching the summit the trio drove back to London, where they dropped the bike off at a docking station.
Filmmaker Ian Laurie, from Kingston, Surrey said: “The only real thing we could do was see if we got the fine or not.
“I never for one second thought that we would be that close.
“We were the last vehicle that was allowed on the Euro Tunnel going back. Then it was ten minutes late.”
The pals pushed themselves to the limit to raise money for Macmillan Cancer Support, in honour of Rob’s father who suffered from the disease.
So far they have already raised an impressive £3000.
Ian said: “The charity thing was very much one of the reasons for doing it. It would be a waste not to raise money.”
In 1967, British cyclist Tommy Simpson died of exhaustion trying to reach the summit of Mont Ventoux during the Tour de France.
The route from the south is 21.8km long and takes 1h30m-2h30m for trained amateur riders.
Professional riders can do it in around one-and-a-quarter hours and the record ascent of 55 mins 51 seconds was set in 2004.
The trio, from Kingston, London, were able to accomplish their mission because they did it when the clocks went back.
Therefore Matt, 41, Ian, 39 and cyclist Rob, 46, actually returned the bike after having it for 24 hours, 59 minutes and 38 seconds.
But because the clocks went back an hour on Sunday October 27, the clock on the Boris Bike stand registered the bike’s return as 3:57.38 am.
Matt said: “I timed the whole thing down to the last second. Bear in mind the times we were driving in Britain were very early in the morning.
“The roads were empty, and on weekends in France, the roads are generally quite clear because there’s no goods traffic.
“But I genuinely think we picked the only day in the entire year that we stood a good chance of completing the challenge.
“Because the clocks went back an hour, it gave us 25 hours rather than 24, which proved all the difference.
“We had the most amazing and fraught journey back because we didn’t know what was happening with the time change.
“We hoped the Eurostar would continue running during that hour they had to repeat, which gave us just enough time to get back to Britain.”
Their prowess has been hailed by London Mayor Boris Johnson, who said: “Huge congratulations to the trio who have conquered Mont Ventoux for charity and shown our Gallic cousins just how sturdy our city’s hire bikes are.
“This was an astonishing feat of endurance made all the more remarkable by the breakneck speed at which they whizzed back in time to avoid getting a fine.”