Quins announced that 23-year-old Lowe has committed his future at the Twickenham Stoop until 2016.
Lowe made his Quins debut in 2009 and has clocked up more than 100 first-team appearances.
He was called up to join England's South Africa tour squad in 2012, when he started against the Southern Barbarians, and is a former England Under-20 international.
"George has incredible experience for someone of his age, and in that respect he is now maturing as a player," Quins rugby director Conor O'Shea said.
"This experience, coupled with George's physical capabilities, should see him move his game on to another level in the coming years.
"It is really exciting for everyone here at Harlequins knowing what he can achieve, and that he wants to commit to doing it here with this club is a great thing."