The 20 year old struck in the 66th minute, unleashing an unstoppable shot from 25 yards after Matty Fryatt had broken the deadlock minutes after the break.
Leicester, League One champions last season, suffered a massive blow in the 8th minute when target man Steve Howard hobbled off to be replaced by Martyn Waghorn.
Leicester were caught out in the 20th minute as Darius Henderson's angled shot dipped viciously, beating Chris Weale before bouncing off the woodwork. Nick Montgomery charged onto the loose ball but fired wide.
The Foxes, playing the more controlled football, almost snatched the lead on 21 minutes when skipper Matt Oakley threaded the ball to Fryatt as on-loan keeper Mark Bunn dived bravely at his feet to block his shot.
United escaped again after failing to cut out Nicky Adams' looping cross from the left. Former Rangers midfielder Dany N'Guessan was given too much space but he squandered the chance mistiming his header as the ball spun over the target.
Jamie Ward, who opened his account in United's 2-0 win over Watford on Saturday, almost broke the deadlock minutes after the break by unleashing a fierce shot from just outside the area which was deflected in spectacular fashion by Weale before Leicester snatched the lead on 52 minutes.
Adams touched a free-kick to Matt Oakley, who drove in a low shot from 20 yards as Fryatt stuck out a foot to beat Bunn to score his first league goal of the season.
United were level in the 66th minute with an electric strike from Treacy as the winger cut in from the left to slam a right foot strike into the top right hand corner.
Substitute Stephen Quinn saw his curling free kick tipped over by Weale and the little midfielder almost snatched the winner for the home side with a 30-yard strike that flew inches over the top with the keeper beaten.