Anthony Knockaert gave the visitors the lead in only the sixth minute before David Nugent made it 2-0 just after the interval.
Richard Chaplow pulled one back for the Lions who came very close to an equaliser in the second half before Schlupp sealed the victory in time added on.
Only six minutes were on the clock when Gary Taylor-Fletcher poked possession free of Shane Lowry's control and, after retrieving the loose ball in the centre of Millwall's half, Knockaert drove untroubled at the Lions defence and fired a low shot home.
The Foxes have won 12 of 14 games when scoring first, and once more highlighted deficiencies in the Sky Bet Championship's worst defence when Taylor-Fletcher skewed wide from close-range after a goalmouth scramble.
But, that chance aside, Millwall were the main aggressor.
Kasper Schmeichel twice punched away well-struck long-range shots from first Nicky Bailey then Chaplow.
Steve Morison was released through on goal and turned Marcin Wasilewski first one way then the other, but the Pole forced him into a wider angle and Schmeichel saved easily.
Scott McDonald also forced Schmeichel into a strong parry, although in doing so wasted a chance to play Morison in on goal.
However, all Millwall's work was undone by another early goal in the second half, this time within three minutes.
Taylor-Fletcher again provided the assist, running onto Lloyd Dyer's pass and luring David Forde wide of his goal, before setting up Nugent for the easiest of his season's 12 league goals - an unmissable, one-yard header.
As in the first half Millwall did most of the huffing and puffing and were finally rewarded with a first goal in four games.
McDonald created the opportunity bustling through the left channel and forcing a reactionary save from Schmeichel. However, Chaplow just reached the loose ball first and slammed home his first Millwall goal.
Millwall's joint caretaker managerial pair, Neil Harris and Scott Fitzgerald, introduced a third striker, but the weakened midfield was left resorting to hopeful long passes.
Jermaine Easter had one glorious opportunity to vindicate his introduction, but fired his shot several rows into the stand.
However, although their caretaker reign remains winless, the duo should be heartened by their side's performance that forced some ragged defending from the league leaders as time wore on.
After clinging on for much of the second half, Leicester entrenched their lead at the Championship's summit late on when sub Schlupp burst through on goal with Millwall committed forwards and slid his shot under Forde.
Leicester do not have QPR's miserly defence, Derby's slick midfield or Burnley's free-scoring strikers, but over the busy festive period they have displayed what their promotion rivals lack - a clinical ruthlessness that wins games.