A double Grand Slam winner with Wales in 2005 and 2008, Henson has been capped 33 times by his country, scoring 133 points, with his last appearance coming in August 2011 against England at the Millennium Stadium.
As well as Head Coach Jones, Henson will also link up again at the Amlin Challenge Cup debutants London with former Ospreys and Wales centre partner Sonny Parker, who joined the Exiles in April.
Jones said. "Gavin has a great talent but over the past three or four seasons he has not realised his full potential for a number of reasons that are well documented. I sincerely hope and believe he can now draw a line under that period and move forward again with London Welsh.
"What's important for Gavin is that he concentrates 100 per cent on his game and learning how to become the best he can possibly be again. I think he can add a great deal to our game and we can also do a lot for him.
"We all appreciate the skills that he can bring to a game but what teammates and coaches will be looking for is consistency of that ability. That's the challenge for Gavin, to hold his concentration fully on rugby as he did when he started his career, when rugby was first, second and third on his list of priorities."
London Welsh Managing Director John Taylor added. "We are obviously delighted to have been able to sign a player of Gavin's pedigree. He is the first to admit things have gone a little haywire over the last few years but he has impressed on me that he still feels he has a lot to prove as a rugby player and is determined to ensure he is not remembered as somebody who never fulfilled his great potential. I am confident he will fit in well here and it is obvious he cannot wait to get back to what he does best - playing rugby."
The Exiles will make their first appearance in the Amlin Challenge Cup next season competing in Pool 5 with Stade Francais Paris, I Cavalieri Prato and Grenoble.