An injury-time own goal by Ben May piled on the misery as Millwall slipped to their seventh home defeat of the season.
There was little May could do to get out of the way as he deflected David Healy's shot past debutant keeper Colin Doyle, who had no chance.
It was a sickening way to lose a match in which Millwall had refused to be bullied by a Leeds side that were quick to put in some bone-crunching tackles, but equally swift to complain when they came the other way.
Den boss Colin Lee handed both keeper Doyle and defender Zak Whitbread their first starts after joining the club on loan from Birmingham City and Liverpool respectively on Thursday.
And the performances of the youngsters were promising, with Doyle producing a couple of excellent saves and Whitbread impressive alongside Paul Robinson and Ady Williams in the home rearguard.
In a scrappy encounter Leeds edged the first half with Rob Hulse and Healy both smacking the woodwork in the opening 17 minutes.
Doyle saved bravely at the feet of the lively Healy, whilst skipper Robbo's superbly-timed tackle prevented Eddie Lewis a shot at goal.
Striker Healy - the only Leeds player to wear a black armband in memory of fellow Irishman George Best - saw his 33rd-minute shot cleared by Tony Craig before keeper Doyle was quickly off his line to smother the ball at the feet of Robbie Blake.
Millwall's first, and arguably best, chance of the game came after 56 minutes when Craig's teasing cross was met by Robinson who saw his header loop across the face of goal and drop a foot wide of Neil Sullivan's right-hand post.
Leeds' American international Eddie Lewis shot wide and then The Lions created their second, and last, real scoring opportunity seven minutes from time. Barry Hayles picked up Craig's cross, swivelled and hit a rasping drive that Neil Sullivan brilliantly tipped over for a corner.
Doyle came to Millwall's rescue with a stunning parry to deny Jonathan Douglas the opening goal after he had fired through a crowd of players as the visitors pressed forward again.
But there was nothing Doyle could do to keep the ball out as Leeds secured maximum points in the cruellest of ways deep into injury-time.

Millwall: Doyle, Ifil, Craig, Williams, Whitbread, P.Robinson (c), Elliott, Dunne, Dyer (May, 75mins), Asaba, Hayles. Subs not used: Vincent, Fanguiero, Cogan, Braniff.
Bookings: Hayles (36mins), Whitbread (46mins)

Leeds United: Sullivan, Kelly (c), Gregan, Healy, Hulse, Lewis, Miller, Derry (Douglas, 37mins), Harding, Kilgallon, Blake (Moore, 90mins). Subs not used: Bennett, Richardson, Walton.
Bookings: Gregan (23mins), Miller (68mins), Moore (90mins)

Ref: Mick Fletcher
Att: 8,134 (Away: 1,365)