United boss Jose Mourinho rang the changes after last week's defeat at Watford, dropping Wayne Rooney to the bench and handing starts to Ander Herrera, Jesse Lingard and Marcus Rashford. Daley Blind was also recalled to left-back by the manager. Foxes boss Claudio Ranieri named Ron-Robert Zieler in goal for the injured Kasper Schmeichel and stayed with the attacking combination of Jamie Vardy and Islam Slimani up front. It marked former United junior Zieler's return to Old Trafford.
United finished the first-half four goals ahead, but it was Leicester whom kicked off the stronger, with the pace of Vardy and Marc Albrighton creating nervous moments for United in the opening exchanges. Indeed, if Albrighton had been sharper with his cross, Vardy could've had Leicester in front inside the opening five minutes.
But it was United and captain for the day Chris Smalling whom took the lead on 22 minutes. A driven corner by Blind was met soundly by Smalling, who saw off Wes Morgan and Robert Huth to put his team 1-0 ahead.
The goal settled United's nerves and with Paul Pogba pulling the strings in midfield, the hosts were irresistible at times in the opening 45.
Their domination was capped with a wonderful team goal on 37 minutes. Pogba and the impressive Jesse Lingard combined to tee up Juan Mata, who fired home from the edge of the area.
Three minutes later and United had their third goal. Again, Leicester were found wanting at a corner as Blind's quick thinking saw Mata collect the ball before firing across Zieler's goal for Rashford to dive in and hit the third.
Two minutes on and it was 4-0 - and again from a Blind corner. This time a more conventional set-piece saw Pogba power a header in after shaking off the attention of Christian Fuchs.
United went into halftime 4-0 ahead for the first time in ten years - and Old Trafford was rocking.
Ranieri reacted by making two changes for the second-half as Andy King and Demerai Gray were thrown on for big name duo Jamie Vardy and Riyad Mahrez.
And it was Gray who lit up the second-half with a wonder goal just before the hour. Collecting possession on the left flank, Gray shook off Lingard's attempts before firing a super strike past David de Gea.
United had allowed Leicester to sneak back into the game with some pedestrian play, but they managed to wrestle back control thanks to Zlatan Ibrahimovic's influence. The big Swede twice came close to making it five, with a volley fired over the top before a snappy low drive being denied by Zieler.
The game petered out, though Mourinho gave Wayne Rooney a late run with six minutes to go.
But it was the first-half which was the story as United fans saw the club's first 45 minutes for some years. For the champions, it was a third league defeat for Leicester, as many as they suffered all season last year.