The 20-year-old played in his side's recent back-to-back victories against Connacht over the last fortnight and is delighted to have extend his deal.
"I am really happy to have re-signed with Quins. I want to be a part of a squad which can win now, but also grow together and become a new force, and I think this is the team to do it with," he said.
"It is a great opportunity for me to keep progressing and with so many young players coming through and challenging for starting places, Harlequins is a really exciting place to be."
Tight-head prop, James Johnston, joined Quins from Ponsonby, New Zealand, in summer 2009. Another young and promising front row, James has played a big part in Conor O'Shea's match day squad since his arrival at the London club.
Having already played two matches for his country Samoa in 2008, the 24 year old has made 31 appearances for the London club and has re-signed until June 2013.
"I was really welcomed into the club two years ago and I have had a great time since. Everyone here has helped me develop my game, especially the more senior players such as Ceri Jones. I want to keep pushing for a starting spot in the match day squad and help Quins win some trophies, something which I really believe is within this team's grasp"
Harlequins are currently second in Pool 1 of the Amlin Challenge Cup just a solitary point behind leaders Bayonne, with the French side still to travel to the Stoop in January.