The 25-year-old has made 29 appearances for the Exiles since joining the Club’s Academy in July 2007 and has already gained recognition at International level with England Saxons.
The athletic forward spent a two years in the Championship before Brian Smith signed him back to the Club for the 2012/13 season.
Exiles Director of Rugby said: “Jon is an extremely gifted back rower. He is a strong carrier, tough in defence and has a fantastic work rate. It’s very rewarding as a coach to see home grown talent kick on to the next level and I believe Jon can launch his international career from London Irish. He is enthusiastic, a great character and epitomises what our squad is about.”
Commenting on his new deal, Jonathan Fisher said: “I am delighted to extend my contract with London Irish. I consider it a privilege to wear the London Irish jersey whenever the opportunity arises. We have a committed, talented and ambitious group of players who are keen to push on over the next few seasons. I plan on being part of a successful team and will do everything in my power to contribute all that I can."