Super sub Chris Taylor stepped off the bench to fire Millwall to their first win in three games and right back into the Play-Off mix.
Taylor, a half-time replacement for Scott Malone, was perfectly placed to sweep home from close range following great work by fellow substitute Andy Keogh.
Both Taylor and Keogh had started the game on the bench, the only two changes to the starting line-up from the team beaten at Ipswich Town last weekend.
In came Dany N'Guessan for his first start of the season up front in place of Keogh, with Malone in for Taylor on the left.
"There's been many really good days at The Den since I've been here which we've enjoyed - and that one was up there," beamed boss Jackett.
"We had to weather the storm; Leicester started at a great pace and we had to defend well. There was a very good save from David Forde and that gave us something to build on for the second-half."
Leicester had clearly been given instructions to try and silence the home crowd with an early goal, and although Nigel Pearson's side enjoyed the majority of possession in the opening half, they failed to register a shot on target.
In fact during a fairly drab first 45 minutes, clear scoring chances were few and far between.
It took a great block by Shane Lowry to prevent Andy King firing Leicester in front on 10 minutes, while Chris Wood glanced a header wide at the other end from James Henry's hooked delivery following Lowry's quick throw.
Henry dragged a low shot wide after Wood had flicked Jimmy Abdou's headed clearance into his path before former Leicester striker N'Guessan went closer still, powering a header from Henry's free-kick over the bar.
Visiting keeper Kasper Schmeichel did well to deny Liam Trotter after he was put through by Wood and Henry fired over from around 25-yards as The Lions began to work their way into the game.
Home keeper David Forde was called into action late in the half using his legs to block David Nugent's goalbound shot as the teams went off all-square at the break.
Millwall made one change with Taylor coming on for Malone and in an enterprising start to the second-half N'Guessan and Lowry - with a spectacular scissor-kick - both saw efforts on goal charged down by The Foxes rearguard.
Wood blasted the ball over the bar from Lowry's hard, low delivery and moments later Danny Drinkwater's 16-yarder flew a yard wide at the other end.
N'Guessan was replaced by Keogh midway through the half and the former Wolves man, who has battled on playing despite having a broken toe, proved an inspired substitution with the deadlock broken just moments later.
Trotter burst into the box after playing a delightful one-two with Keogh pulled back a low pass and Taylor was perfectly placed to fire home from virtually three yards.
Jackett made his second change 11 minutes from time with Shaun Batt replacing Wood, who had worked tiressly upfront all afternoon.
And it was Batt who twice had great opportunities to add further goals late on.
Leicester went for broke in the final minutes trying to grab an equaliser and inevitably left themselves open on the break.
Unfortunately Batt was unable to capitalise on both occasions, but with Man of the Match Danny Shittu and the rest of the team defended resolutely, Millwall held firm for a three point haul that moves the team back into the top six.
Forde, A.Smith, Lowry, Abdou, Shittu, Beevers, Malone (C.Taylor, 46mins), Trotter, N'Guessan (Keogh, 66mins), Wood (Batt, 79mins), Henry. Subs not used: M.Taylor, J.Smith, Feeney, Wright.
Abdou (20mins), Shittu (82mins)
Leicester City: Schmeichel, De Laet, Konchesky, Drinkwater, Morgan, Whitbread, Vardy, King (James, 70mins), Dyer (Futacs, 70mins), Knockaert (Marshall, 63mins), Nugent. Subs not used: Logan, Schlupp, Moore, Lingard.
Bookings: Nugent (20mins), Knockaert (34mins), Futacs (85mins)