Thorpe, who made 127 appearances for Exiles after joining the academy in 2002, was poised to stay on at the Madejski Stadium under former head coach Toby Booth.
But instead the 27-year-old has become the fourth player allowed to leave by new director of rugby Brian Smith after Jonathan Spratt, Tom Smith and Ross Samson, who is reported to be moving to Championship side London Scottish.
Thorpe will follow former Exiles Dan Bowden and Adam Thompstone to Welford Road and said in a statement: "I would like to thank all players and staff that I have been involved with over my time at London Irish and also the tremendous supporter base.
"I will look back at my time at Irish with fond memories. I am now looking forward to a new challenge."
Although Leicester are yet to confirm the move, a message from Thorpe on his twitter account, which has subsequently been deleted, made the news public.
"Thanks for a great decade of rugby London Irish. Now for a new challenge . . . signed with Leicester Tigers this afternoon #happylad," read the message.