Former Bath player Catt joined Irish in 2004 and has been one of the Guinness Premiership club's most influential players since, playing 60 games and scoring 66 points.
He even earned a recall to the England squad this year and captained his country in two matches in the RBS 6 Nations Championship.
Catt said: "I've enjoyed my rugby at London Irish and am really looking forward to the change of role.
"For me moving into coaching is a natural progression and I'm pleased to have the opportunity of doing so with London Irish.
"We've a very talented set of young backs with the potential to make a real impact at European and international level, it will be rewarding to play a part in helping them to get there."
Irish director of rugby Brian Smith added: "Mike's contribution to this club over the past three seasons has been enormous.
"He is a world-class player who has shown that he has the skills to continue to compete at the highest international level."