Jose Mourinho’s second-half replacements worked a treat as goals from substitutes Marcus Rashford and Marouane Fellaini proved the difference.
United were utterly dominant in the opening stanza but failed to pry open a resolute Leicester defence.
The Red Devils did have the ball in the back of the net before the break courtesy of Jaun Mata but his effort was chalked off due to a marginal offside.
Paul Pogba had some speculative attempts on goal but Kasper Schmeichel was largely untroubled.
After the break the Foxes looked sharper on the break and had United worried on a few occasions.
The home side were eventually afforded the break they sought after Danny Simpson handled in the area and the referee pointed to the spot.
Romelu Lukaku stepped up to take the spot kick but Schmeichel guessed the right way to save the burly Belgian’s effort.
The deadlock was eventually broken on the 70 minute mark as substitute Rashford swept home from a corner.
United sealed the points thanks to Fellaini in the 81st minute as he diverted Jesse Lingard’s shot into the net.