World Cup winner Catt, 40, will work alongside new England head coach Stuart Lancaster on a five-match tour of South Africa in June.
His final game with Exiles will be this Saturday's home clash with Gloucester (2pm ko).
"It has been a difficult decision and I will be sad to leave London Irish," said Catt.
"I want to say a huge thanks to all the players and coaches I have worked with since 2004.
"Toby Booth in particular deserves massive credit for what and how he has taught me. Thanks also to the supporters who have been brilliant over the last eight years and I will take away some lasting memories."
Andy Martin, London Irish's chief executive, added: "On behalf of everyone at London Irish I would like to thank Mike for his eight years as a player and coach with the club.
"He has made an outstanding contribution during his time with us both on the field and as a role model in sharing his extensive knowledge and experience. We are disappointed to see him leave, however, we completely understand that he has been offered an opportunity that he could not turn down and we wish him and his family all the very best for the future."