A George Byers brace, either side of half-time, bookended Kenji Gorrè’s long-range effort for the Swans to hand City with their first loss since early February, despite a late Sam Hughes strike.
The Foxes pushed hard to regain a foothold in the game, although the Swans dug deep to see out the result, even after defender Mael Davies saw red with two minutes to go.
City’s starting XI included two changes from the 4-0 success over Liverpool last Monday, with Max Bramley and Lamine Kaba Sherif replacing Daniel Iversen and Fousseni Diabaté.
Steve Beaglehole’s squad made an industrious start, showing strength in the middle and a creative spark on the flanks, leading to their first chance after just six minutes.
Harvey Barnes, cutting inside his man on the left, spotted the run of George Thomas on the opposite side, lofting it over the Swans backline, but his colleague’s shot was blocked well.
While Byers tried his luck from distance for the hosts, Bramley had little to do in the opening proceedings, watching on as City’s front-three caused Swansea problems.
Josh Gordon, too, was proving to be a handful and, after 13 minutes, it was his shot, from outside the area, which forced home ‘keeper Steve Benda into his first save of the night.
Despite all of City’s good work, they were behind on the half-hour mark when Botti Biabi located Byers on the edge of the penalty area, enabling him to lash it into the top of Bramley’s net.
Leicester's plight worsened, too, with seconds of the first half remaining when another fine strike from Gorrè flew into the net, giving Leicester little time to respond.
Beaglehole’s side certainly posed more of an attacking threat after the interval, but it was Swansea’s Gorrè who carved out the first chance, prompting Bramley into a smart low save.
City had the Welsh outfit penned in for large periods in the second half, but they lacked the cutting edge required to get back into the game.
With 20 minutes to go, they were punished further when Gorrè galloped down the left flank and found the head of Byers, who nodded it past Bramley for his second of the night.
The arrears were reduced by Hughes late on, getting the final touch on Kiernan Dewsbury-Hall's corner.
A night of drama wasn't over, though, when Swans defender Davies was shown a straight red card for a late challenge on City substitute Liandro Martis on the left flank.
Major moment…
While City will have been frustrated to go behind in the 31st minute, allowing the Swans to double that advantage before the break ultimately cost them dearly in Landore.
It galvanised their Welsh hosts and doubled the height of the mountain they had to climb in the second period, which eventually proved to be too great a gradient for them to scale.
Where does it leave us?
How the top of the #PL2 table looks following tonight's result... 📊#lcfcu23s pic.twitter.com/A86lCA1iis
— Leicester City (@LCFC) March 12, 2018
City remain top of Division 1, but miss out on the chance to double their lead to six points, meaning the chasing pack, including Liverpool and Arsenal, could yet catch them over the coming weeks.
Coming up…
Leicester park their Premier League 2 duties for a Premier League Cup trip to Southampton on Friday in the Last-16 stage (7pm kick-off) before going to Sunderland next month.
City are on 33 points, three above Liverpool and four above Arsenal, although they possess one and two games in hand respectively.
The details…
Swansea: Benda, Davies, Lewis, Dulca, Harries (c), Cooper (Cabango 33), Garrick (Cullen 59), Marić, Biabi, Byers, Gorrè (Blake 89)
Subs not used: Zabret, De Boer
Goals: Byers 31, 70 Gorrè 45+2
City: Bramley, Johnson (Martis 59), Wood, Choudhury (c), Knight, Hughes, Thomas (Pascanu 73), Dewsbury-Hall, Gordon, Barnes, Kaba Sherif (Eppiah 45)
Subs not used: Stolarczyk, Wright
Goal: Hughes 80
Referee: Savvas Yianni