After Paul Gallagher had fired the Foxes into an early lead, United winger Mark Yeates popped up with two first-half goals of his own – the second a glorious 25-yard free-kick – to turn the match on its head.
United looked set to hold on for a valuable victory as a dogged defensive display kept the hosts at bay after the interval.
But Yeates was then penalised for handball in the area by referee Phil Gibbs – a decision the visitors strongly protested against – and sub Howard made no mistake from 12 yards to earn Leicester a share of the spoils.
The Blades remain 21st in the table as a result, while Leicester move up one place to 17th.
Foxes boss Sven-Goran Eriksson made one enforced change to his starting line-up, with Jack Hobbs coming in for the injured Miguel Vitor.
Andy Reid made his first start for United in place of Jamie Ward, who was ruled out by an ankle problem, while Stephen Jordan replaced Johnny Ertl in defence.
Martyn Waghorn forced an early save from Blades goalkeeper Steve Simonsen at his near post before Leicester took the lead in the eighth minute.
Waghorn was brought down by Kyle Bartley on the edge of the box and Gallagher stepped up to curl the resulting free-kick across Simonsen and into the top-left corner.
A flowing move almost presented Leicester skipper Andy King with the opportunity to add a second nine minutes later, but Kyle Naughton’s over-hit pass allowed Simonsen to gather at the midfielder’s feet.
It took United until the 28th minute to register their first shot on goal as Yeates curled a tame effort into the arms of Chris Weale.
But seconds later, they were level as a quick break saw Leicester exposed at the back.
Reid and Yeates were left two-on-one against Hobbs and the on-loan Sunderland winger carried the ball into the area before squaring to leave his team-mate with a simple finish.
The goal lifted the visitors and they took a shock lead four minutes before the break as Yeates scored his second of the game.
Curtis Davies was adjudged to have fouled the former Middlesbrough midfielder 25 yards from goal and Yeates made him pay by finding the top-right corner with a wonderful curling free-kick.
Leicester were unable to threaten the Blades goal in the early stages of the second period and Eriksson responded by introducing Steve Howard and Lloyd Dyer in place of Waghorn and Darius Vassell.
A brief period of pressure followed from the home side, but Simonsen remained relatively untroubled in the United goal.
The former Stoke stopper was called into action in the 68th minute to keep out another Gallagher free-kick from just outside the area.
Yuki Abe and Howard then saw low shots saved by Simonsen and King hooked a hopeful effort wide as Leicester continued to press for an equaliser.
And they were finally rewarded in injury-time as Yeates was adjudged to have handled and Howard drilled his penalty into the bottom-left corner.