The 25-year-old came through the club's academy and made his debut back in 2006. He has gone onto make 18 European appearances in that time and is delighted to have secured a new contract.
"I see us being a dominant side over the next few years and that's something I want to be a part of," he said.
"I love playing rugby for this club and I'm always proud to pull on the Harlequins jersey."
"I want to continue being part of a successful team, in a positive environment, whilst striving to fulfill my own international aspirations."
Harlequins director of rugby Conor O'Shea added, "George is one of the most competitive people I have ever come across.
"As someone who has grown up in the club he has ambitions over the coming years to win silverware, not only with us but also internationally.
"We are delighted he has committed to us for the best years of his rugby career and look forward to helping him achieve his goals."
Harlequins kick off their Heineken Cup campaign this year by hosting Connacht on Friday November 11.