Thatcher had a clause in his contract that allowed him to talk to Premiership clubs following the Foxes relegation and has joined Man City for an undisclosed fee.
"There were rumours and a number of clubs had spoken to my agent just to find out what the deal was with the release clause in my contract with Leicester," he told the official Man City webite.
"But Manchester City was a concrete offer so I came here to sign.
"Until I signed for Leicester last year I had played all my football in London but it has not been a problem for me moving north.
"I played in the new stadium last year in the League game with Leicester and it is a fantastic place to play football."