De Laet, 23, and James, 20, join the Foxes for an undisclosed fee, on three-year deals running until June 2015.
Leicester manager Nigel Pearson revealed the club had been monitoring the pair for "some time" and said he was "very pleased" to wrap up the deal.
“Both are hungry, young players with winning mentalities, experience of Championship football and a desire to be successful," he said. "They will add considerable quality to the squad.”
Both players leave Old Trafford following frustrating campaigns: Belgian defender De Laet had his season-long loan deal at Norwich City terminated in January after just seven first-team appearances before spending the rest of 2011/12 in United’s Reserves, while James struggled with injury for much of the season, managing just five appearances for Warren Joyce’s second-string side.
Defender De Laet arrived at United as a surprise signing from Stoke City in January 2009. He only