It may have been a different story had Steve Morison converted a third-minute penalty, but the Millwall marksman blazed his spot-kick high and wide.
Lloyd Dyer quickly punished the Lions by marking his 100th appearance for Leicester with the opening goal but that was quickly cancelled out by James Henry's superb strike.
Ayegbeni Yakubu brought a touch of Premier League class to proceedings with a slick finish to restore the Foxes' lead before Sol Bamba gave Eriksson's side a two-goal cushion at half-time.
Jack Smith momentarily got Millwall back into it midway through the second half, showing Morison how it should be done from the penalty spot.
But their hopes were soon dashed with the 72nd-minute sending-off of debutant Jens Berthel Askou, before Bamba grabbed his second goal of the game two minutes later to inflict a first defeat in 10 league matches on the Lions.
Yakubu and Darius Vassell were paired together for the first time as Eriksson recalled the strikers as two of three changes.
Yakubu was cup-tied against Manchester City in midweek while Vassell has missed the last two matches, including Yakubu's debut at Preston, with a hamstring injury.
Eriksson also handed Manchester City loanee Ben Mee his Leicester debut.
Millwall also made three changes. Berthel Askou made his debut after joining on loan from Norwich, Theo Robinson started in attack for the first time since completing his permanent switch from Huddersfield and Chris Hackett was also recalled.
Mee made a disastrous start, bringing down Robinson with a late challenge just inside the area. However, Morison's woeful spot-kick let Leicester off the hook.
Former Millwall trainee Dyer then made his former club pay five minutes later, collecting Yakubu's pass and firing a shot into the bottom corner from 12 yards.
But the lead was shortlived as Henry's 20-yard thunderbolt immediately levelled things up.
Dyer repaid the favour to Yakubu after 22 minutes. The Everton striker collected the ball and picked his spot beyond David Forde.
Millwall almost equalised again but Alan Dunne's header was tipped over by Chris Weale. At the other end, Forde denied Dyer.
Yakubu turned provider again when Bamba converted his cross from close range after 35 minutes.
Hackett and Nadjim Abdou threatened for the Lions early in the second half before Mee conceded a second penalty, this time clipping Hackett's heels inside the box. Smith squeezed the ball past Weale, who went the right way.
The drama then came thick and fast. Berthel Askou was shown a straight red card for a two-footed lunge at Yakubu.
From the resulting free-kick, Bamba bundled Richie Wellens' cross over the line.
Yuki Abe shot just wide soon afterwards as Leicester looked to extend their lead, and substitute Steve Howard did likewise in stoppage time.