Brighton & Hove Albion 0 Sheffield United 1
Jagielka's first-half strike secured a seventh win out of eight for the Blades and ended Brighton's unbeaten three-match run.
Blades boss Neil Warnock turned down a £4million bid from Wigan for Jagielka in the summer.
That decision looks a wise one now as the centre-half turned midfielder earned United three more precious points in their quest to reach the Premiership.
Jagielka struck the decisive goal in the 25th minute after an exchange of passes with the prolific Steve Kabba put him clear inside the Brighton box.
His first shot was blocked by Brighton's on-loan keeper Wayne Henderson, but Jagielka found the roof of the net with his follow-up as he fell.
It was no more than United deserved for their early enterprise. They could have been in front sooner, Alan Quinn forcing Henderson to save at his near post with an angled drive on the break and Keith Gillespie firing just wide via a deflection.
Not much was seen of Brighton's young and lively four-pronged attack in the opening 45 minutes.
The closest they came was when Jake Robinson stretched to reach a cross from Leon Knight but was unable to direct his effort on target.
Brighton improved after the break and Paddy Kenny had to rush to the edge of his area to clear from Knight as he chased a long ball by Frenchman Sebastien Carole.
Kenny later saved a free-kick from long range by Richard Carpenter at the second attempt.
Brighton's best chance of an equaliser came with three minutes left when midfielder Dean Hammond headed inches wide from Knight's cross.
Knight also spurned a late chance to salvage a point when he blazed wide.
Referee Lee Probert incurred the wrath of the crowd by booking a total of nine players, five of them for the home side, and lecturing Brighton physio Malcolm Stuart in the second half after consulting with his fourth official.