Thompstone joined the London Irish academy in 2007 and made his way through the ranks, making his senior debut in 2008 and has also been capped at England Under 20 level.
Commenting on the decision to extend his contract Thompstone said: "I am delighted to be staying with London Irish for another year. I have thoroughly enjoyed my time here and want to use the remainder of this year and next year to get back to my very best form, and help the side challenge for silverware."
London Irish head coach Toby Booth said: "We are thrilled that Adam has signed for a further season. He is a strong, quick, agile player and one of the best finishers in England. The Aviva Premiership is incredibly competitive so it is therefore imperative that you hold on to your best young players, so to have Adam re-sign is great news for the club. "