The mystery punter is said to have been "hopping up and down" outside the chain's branch at Waterloo station, where Eurostar trains linking London and Paris start and terminate, waiting for staff to open up.
He then handed over a stash of Euros before rushing off to catch the train back to France, leaving the manager to chase him with the betting slip.
"The man was clearly in a hurry to catch the train back. He told us that he would be back on July 10 (when the World Cup final is held) to collect the winnings," said William Hill spokesman Rupert Adams Tuesday.
"We didn't have the heart to tell him that he could have placed the bet on either the telephone or on the Internet."
The man staked 10,000 euros at odds of 11 to four and will collect 37,500 euros if Brazil lift the trophy.
Unlike Britain, where wagers can be placed at a number of privately-owned chains, betting in France is more tightly controlled with two franchises, one running bets on horseracing, the other lottery and scratchcard games.