The 53-year-old actor promised Christy Turlington that he would take part in the 2016 race if she finished this year's marathon in under four hours.
Unluckily for George, the Calvin Klein model completed the 26.2-mile course in three hours and 45 minutes yesterday (26.04.15), meaning the 'Interstellar' star will have to have to accept his forfeit, whilst wearing a T-shirt for Christy's charity Every Mother Counts.
Speaking before the race, Christy told The Daily Telegraph: "Thinking about George running a marathon and having a good night's sleep before the race will help my time. The moment I cross the line, I want to go for a pint."
Other celebrities who took part in the London Marathon were Radio 2 DJ Chris Evans, TV presenter Jenni Falconer, Formula One driver Jenson Button, Radio 1 DJ Greg James and double Olympic gold medalist rower James Cracknell.
Greg, who completed the race in 4 hours and 26 minutes, took to Twitter to express how "proud" he was.
He wrote: "Done. 4:26. Very proud. Incredible crowd - thank you for shouting at me all the way round".
He later added: "Does anyone want to run the marathon on behalf of me next year to cover the cost of the stair lift I've had to buy because of yesterday?"