Stefan Maierhofer's first-half brace hoisted Millwall out of the bottom three and gave Ian Holloway's squad renewed belief that The Lions can preserve their Championship status.
Austrian international Maierhofer picked the perfect time to score his first goals in a blue shirt, providing textbook headed finishes on 16 and 30 minutes from Lee Martin deliveries.
Although Emmanuel Ledesma reduced the deficit with 10 minutes remaining to set up a nail-biting finale, the final scoreline really did not do justice to how well Holloway's charges had played.
The manager had been able to name an unchanged side for a second successive game, but the match was just six minutes old when newly crowned Player of the Year David Forde prevented Boro from breaking the deadlock, getting a hand to push Danny Graham's shot onto the post after Shaun Williams had lost possession in his own half.
At the other end skipper Alan Dunne had The Lions first opportunity, but he headed wide under pressure from Martin's deep corner a couple of minutes later.
A long ball from Mark Beevers was picked up by Maierhofer, who squeezed between two defenders and dispatched a low cross that was deflected for a corner. Boro failed to clear the danger adequately enabling Martin a second bite at the cherry and his cross was headed into the side netting by the towering Austrian.
Maierhofer was not to be denied however, and when Martin again sent over a delicious delivery from the left on 16 minutes he rose to power a textbook header into the roof of the net, racing over to the dugout to hug kitman Adrian Wisson, celebrating his birthday today.
Graham forced Forde into another save midway through the half when the keeper scrambled low to his left to gather a header after Lee Tomlin had got behind the defence to cross from the byline.
Millwall created another chance in the 27th-minute, this time Dunne's long clearance was expertly flicked on by Maierhofer into the path of Martin and his low cross was shanked for a corner by Jonathan Woodgate.
Although the home defence managed to clear that, they were breached for a second time just three minutes later. Martin was the provider with an inviting cross that Maierhofer met perfectly at the far post to send a downward header beyond the reach of keeper Dimi Konstantopoulos.
Martyn Woolford twice went agonisingly close to putting the game beyond Boro's reach. On 29 minutes a magnificent defence splitting pass from Williams was picked up by Woolford, but his goalbound shot was blocked and then moments later he stumbled as he was about to pull the trigger from eight yards.
Having established a two-goal cushion by the break, The Lions knew that Boro would be looking to get back into the game as early as possible to give them a chance of salvaging something from the game.
Within seconds of the restart that so nearly happened when the ball ricocheted in the Millwall area before the danger was cleared and then Ledesma's curling cross-shot dropped onto the roof of the net.
The Lions dug in, though, and weathered a nervy 15-minute spell at the start of the second-half and they began to create further chances of their own. Morison had a golden opportunity to add a third goal when he latched onto a ball over the top from Dunne, bore down on goal and then hit his shot straight into the hands of the keeper.
Holloway's men went even closer just moments later when sub Morison, a 62nd-minute replacement for Williams, expertly laid he ball into the path of Woolford in a central position, and his crisp strike was superbly tipped away at full stretch by Konstantopoulos.
Boro clawed a goal back on 79 minutes with a goal that infuriated the visiting bench. There seemed to be no contact when Graham went to ground a yard outside the penalty area and up stepped Ledesma to dispatch a smart chipped effort over the wall and under the bar.
The home side chucked everything they had at Millwall and The Lions dug in once again to protect the narrowest of leads. But it was Holloway's men who were denied a third goal in stoppage time as former Boro striker Scott McDonald drilled in a low 20-yard shot that Konstantopoulos blocked and Martin poked home the rebound. Up shot the assistant linesman's flag to rule out his effort with the narrowest of offside decisions.
In the end justice was seen to be done though and this win - Millwall's third consecutive three-point haul away from SE16 - sets up a massive Den clash against Doncaster on Monday.
Middlesbrough: Konstantopoulos, Omeruo, Chalobah, Tomlin, Adomah, Butterfield (Williams, 61mins), Woodgate, Ledesma, Graham, Whitehead (Friend, 85mins), Leadbitter (Main, 73mins). Subs not used: Steele, Varga, Hines, Morris.
Booking: Tomlin (59mins)
Scorer: Ledesma (80mins)
Millwall: Forde, Edwards, Malone, Bailey, Beevers, Dunne, Martin, Williams (Morison, 62mins), Garvan (McDonald, 85mins), Maierhofer (Abdou, 72mins), Woolford. Subs not used: Bywater, Robinson, Easter, Jackson.
Bookings: Malone (71mins), Martin (74mins), Forde (80mins)
Scorer: Maierhofer (16, 30mins)
Att: 15,342 (away 335)