The former Cambridge University player rose through the ranks at Franklin's Gardens from the club's academy and will link up with his new side in the summer.
"Joe hits the mark for us. He is a very talented player with a great future ahead of him - he is quick and powerful and I believe he will fit nicely into our set up and with the style of rugby we play," said London Irish head coach Toby Booth.
"His versatility on the field makes him a valuable asset to have on board and further boosts the strength in depth that we already have.
"We're an ambitious club and continuing to bring in high calibre players, such as Joe, is key to giving us the edge and helping us to achieve our goals."
Ansbro, made five appearances for the Saints in the Heineken Cup pool stage this year, helping them to top Pool one and clinch a quarter final place and the 25-year-old is looking forward to his move south.
"Toby has a terrific coaching record at London Irish and he is someone I look forward to working with. There are some great players who have committed their long term futures to the club and that speaks volumes for what is happening there," he said.
The Saints will line up against Ulster in the knock-out stages on Sunday April 10 at stadium:mk