Iheanacho maintained his red-hot scoring streak as he bagged a brace against the Red Devils, who put in a rather drab display.
Leicester City reached the FA Cup semi-finals for the first time since 1982 after beating Manchester United 3-1.
It was a much-deserved win for Leicester, who will now dare to dream to reach their fifth FA Cup final when they face Southampton next month.
An attempt to play out from the back had disastrous consequences for the visitors. Harry Maguire’s pass put Fred, who was in the box and facing his own goal, in a tight spot. Fred tried to pass back to Dean Henderson but the ball was stolen by Kelechi Iheanacho, who blasted it inside the net to put his side on the front foot.
Mason Greenwood leveled the scoreline eight minutes after the half-hour mark, firing a first-time shot into the back of the net.
Seven minutes after the break, Youri Tielemans blasted a low shot into the bottom corner to restore the Foxes’ lead. Iheanacho, who had netted a hat-trick in his previous match, then headed home from a corner to take his tally up to six goals in his last four games.
Leicester City: Schmeichel – 6, Fofana – 8, Evans – 7, Soyuncu – 8, Albrighton – 7, Tielemans – 8, Ndidi – 8, Castagne – 7, Perez – 6, Iheanacho – 9, Vardy – 5
Manchester United: Henderson – 6, Wan-Bissaka – 5, Lindelof – 6, Maguire – 4, Telles – 6, Matic – 4, Fred – 3, Greenwood – 6, Van de Beek – 6, Pogba – 6, Martial – 5, Shaw – 6, McTominay – 4, Fernandes – 6, Cavani – 5, Diallo – 5