Paul Gallagher, nicknamed 'Jedward' by his team-mates who reckon he's a dead ringer for the warbling twins from X-Factor, stole the show with a hat-trick.
It was the first time City had scored five goals or more since their move to the Walkers Stadium in 2002.
It was also the Foxes' biggest win since they beat Sunderland 5-2 at Filbert Street in March 2000 with three goals from Stan Collymore and Emile Heskey.
There were chances for both sides inside the first two minutes when Lloyd Dyer drove straight at the keeper from 10 yards before Chris Weale denied Gary Hooper after Scunthorpe had broken quickly.
But it was City who made their next chance count with Michael Morrison heading home Martyn Waghorn's in-swinging corner in the sixth minute.
And the Foxes doubled their lead on 16 minutes when Matty Oakley and Dyer combined to set up Gallagher, who scored with an 18-yard shot which left the keeper stranded after it deflected off Scunthorpe defender Marcus Williams.
United were denied a way back on 20 minutes when Weale twisted in mid air to palm away a goalbound header from Rob Jones.
City then put the game beyond the visitors with a third goal on 25 minutes when Gallagher curled a 18-yard free kick around the defensive wall and inside Joe Murphy's far post.
And it was four for Leicester on 34 minutes when a half-cleared Gallagher cross fell to Waghorn, who had time to control the ball before lashing it home from 10 yards.
Scunthorpe pulled a goal back on 55 minutes when Sam Togwell helped on Paul Hayes' goalbound shot.
But Gallagher scored his third and Leicester's fifth on 75 minutes when, from a Richie Wellens cross, he looped a clever back-header over the advancing keeper.