With closest rivals Derby County being held to a 0-0 draw at Coventry City on the same afternoon, Steve Beaglehole’s in-form side now have an unassailable lead in first place and will advance to the play-offs.
Paul Gallagher got the scoring underway in the 35th minute with a thunderous strike into the top corner from 25 yards, and followed it up six minutes later with a glancing header from a Richie Wellens cross to make it 2-0.
Substitutes Harry Panayiotou and Jacob Blyth then combined to secure the victory with a late third - the latter's looping header finding its way over the goalkeeper for a deserved 3-0 victory.
Wellens, Gallagher and Waghorn all started as the three overages players for The Foxes, who were captained by centre back Alie Sesay in a 4-3-3 formation that saw Waghorn supported by Tom Hopper and England Under-18 international Joe Dodoo in attack.
The hosts meanwhile included striker Reece Hales in the starting XI, who was on target for the Blues when the sides met at Bevoir Drive earlier in the campaign – Leicester coming from behind to win 2-1 thanks to goals from Conor Clifford and Hopper.
It was Beaglehole’s energetic Foxes who were first to test the opposing goalkeeper inside five minutes, when Clifford fired at goal from 30 yards after Waghorn’s snap-shot was deflected out for a corner.
Waghorn again was involved in the 13th minute, attacking a bouncing ball in the area that Jack Deaman hesitated over, only to see Callum Preston get his body behind the shot and gather at the second attempt.
Birmingham striker Hales was then replaced by Liam Truslove on 17 minutes, who had picked up what looked like an ankle injury, while at the other end Gallagher stung the palms of Preston with a 20 yard drive.
Dodoo was then denied by a well-timed challenge in the area around the 30-minute mark after Clifford’s stinging goal-bound effort was blocked by defender Deaman.
With ten minutes until the break, a moment a real quality from Gallagher gave Leicester a deserved lead.
The midfielder picked the ball up in a central position and drove a powerful right footed shot at goal, which swerved left to right before nestling in the top corner, making it 1-0 and giving Birmingham ‘keeper Preston no chance.
Frustrated, Brum defender Pablo Ibanez went into the book for a nasty challenge on Hopper, with Gallagher forcing an excellent save from the free kick.
And from the resulting corner, City doubled their lead. The ball found Wellens on the left, who put an excellent left footed ball in for Gallagher to glance a sublime header into the same corner as his first goal.
Ibanez made way for Graham Hutchison as City got the second 45 underway with another Gallagher effort from range – but this time Preston was able to make a comfortable save.
Despite a bright start to the second half from Leicester, the home side had the ball in the net on 53 minutes when Dominic Dell picked up a rebound from Amari’i Bell’s shot to finish into the corner, only to see the flag go up for offside.
Just before the hour mark City had a wonderfully worked chance to take a 3-0 lead. Wellens took a quick free kick over the top into the run of Waghorn, but his first time effort on his left foot cannoned over the crossbar.
Beaglehole then introduced Jacob Blyth for Hopper in the 62nd minute, fresh from his two-goal substitute appearance against Derby last time out, while for Birmingham Charlee Adams shot over from a good position.
Second year scholar Panayioutou then joined the attack at the expense of Dodoo as City went after more goals to kill the game off.
And they almost got one with 15 minutes left to play, as Gallagher was felled on the edge of the box, with Waghorn curled the resulting free kick onto the crossbar.
City kept up the pressure with Wellens seeing an effort cleared off the line after Preston had spilled a Gallagher effort into his path.
The hopes of a Birmingham comeback were looking increasingly unlikely as the game entered the final five minutes, especially as the combination of Wellens and Gallagher was causing so many problems for the home side.
The Foxes then put he icing on the cake in the final two minutes of normal time, when substitute Blyth's looping header found its way over Preston from a Panayiotou cross to make it 3-0, sealing a deserved win.
Birmingham (4-4-2): Preston, Higgins, Bell, Lee, Deaman, Ibanez (Hutchison 46), Adams (Brown 72), Gnahore, Dell, Hales (Truslove 17), Solomon-Otabor
Unused subs: Hutchison, Trueman, Gray
City (4-3-3): A. Smith, Anton, Elder, Clifford, Sesay, Taft, Gallagher, Wellens, Hopper (Blyth 62), Waghorn, Dodoo (Panayiotou 68)
Unused subs: Watson, J. Smith, Cain
Goals: Gallagher 35, 41, Blyth 88