United swept ahead in the 12th minute, with a big question-mark hanging over the decision to award the Blades a penalty.
Billy Sharp chased a hopeful ball towards the area, shadowed by full-back Patrick Kisnorbo and the striker appeared to stumble as the Leicester skipper made contact.
After consulting his assistant referee Mike Pike pointed to the spot and red-carded Kisnorbo for a professional foul .Paul Henderson blocked Beattie's spot-kick, but could not hold on to the ball as the striker made sure at the second attempt.
Beattie added the second in the 14th minute after Richard Stearman had fouled Sharp, lashing in strike from 30 yards as Henderson blundered, letting the ball slip from his grasp and spin over the line.
The third came in the 19th minute, the Leicester's back four gave Beattie too much space as Michael Tonge pumped in a corner from the left and he finished off with a far-post downward header.
Leicester were confined to long-range strikes, James Wesolowski having a shot blocked before Steve Howard went close, his shot from 22 yards flashing inches wide of the target.
The former Derby striker was inches off target with a header on the hour from a Joe Mattock free-kick as Leicester looked for a way back, but the Foxes rarely looked like making a game of it.
Substitute Jon Stead, on for Beattie, was foiled by James Chambers late in the game after neat work from Sharp, as United almost added to their tally against the shell-shocked visitors.