Full-time score from the Riverside Stadium
Martyn Woolford picked the perfect time to score his first goal for Millwall as The Lions ended a run of five successive defeats with a battling display to take three vital points at Middlesbrough.
Boro defender George Friend had already put through his own net on 25 minutes when Woolford stole in to fire Millwall 2-0 ahead after home keeper Jason Steele had parried a shot by Benik Afobe.
Boro produced a second-half onslaught and reduced the arrears with 15 minutes remaining courtesy of Curtis Main and although the home side had a last minute goal by sub Scott McDonald rightly ruled out for offside, it was Kenny Jackett's side who held on for a morale-boosting win.
"We showed flashes of quality," enthused Jackett afterwards. "We made changes to the side and in Benik Afobe and John Marquis had young players up front, and I thought they did well.
We played well in the first half and got the goals, and in the second half we got into some good positions on the break too.
After the defeats we've had in the League this was a very good win for us and excellent to get a double over a team like Middlesbrough.
There's a lot of games to come and anything is possible. The spirit we had today was good and we will be looking to move on from this victory in a positive way now.
We will need a real good run like we had earlier in the season to get up there, but we will see what happens."
Lions boss Jackett rung the changes, making five alterations to the team that started against Peterborough on Tuesday night.
Back came skipper Danny Shittu from injury and illness and right-back Alan Dunne in a new-look rearguard that saw Mark Beevers switch to left-back in place of Shane Lowry, who was serving a one match ban.
Woolford returned to left midfield, with Josh Wright partnering Jimmy Abdou in the middle and a fresh strike partnership of Afobe and Marquis.
With both teams struggling in the league recently, the early stages had a slightly tentative feel to it as the sides prised and probed for an opening, but without really creating any clear chances.
Karleigh Osborne was somewhat harshly booked in the 15th minute for a foul on Curtis Main when the Boro man appeared to stumble over the ball around 20 yards out. Thankfully there was to be no further punishment for The Lions as Grant Leadbitter curled the resulting free-kick two yards over.
After a quiet opening it was Millwall who got their nose ahead midway through the half when James Henry despatched a deep delivery from the left and Friend turned the ball into his own net under pressure from Marquis.
Woolford doubled Millwall's tally making it 2-0 five minutes before the break with a clinical finish after Afobe burst between two home defenders and unleashed a stinging drive that keeper Steele could only parry into his path. Woolford did the rest, driving the ball beyond Steele's reach.
With a two-goal cushion and seeming to be in control, a double substitution by Boro at the break ensured that the second half had a different look to it.
Boro pushed forward and as a result, Jackett's men dug deep to protect their lead with a real battling, backs to the wall performance.
Leadbitter went close with a fierce volley that flew over before sub Ishmael Miller wasted a decent chance, firing over from a central position.
Lions keeper David Forde was called into action making a smart one-handed stop to push Mustapha Carayol’s low shot behind for a corner and then defender Osborne blocked Main’s shot for another corner as Boro piled on the pressure.
At the other end the lively Afobe, who was a constant threat, dragged his effort wide after holding off former Lion Friend, whilst Henry lashed the ball into the side netting following good work by Dunne on the right.
Marquis rushed at his shot from Henry's astute delivery, and keeper Steele comfortably collected the striker's scuffed effort.
Just seconds later play had swung up the other end and Main converted from close range after he was picked out by a cross from the left from Friend.
Boro were spurred on and The Lions had Forde to thank for a smart stop to smother the danger from sub McDonald's low shot.
Millwall breathed a sigh of relief five minutes from time when Main squandered a glorious opportunity to equalise, but contrived to fire wide with the goal at his mercy.
Afobe could count himself unlucky to have his appeal for a penalty waved away as he was bundled over by McManus after bursting into the area, but his plea fell on deaf ears.
Back came Boro and in a frantic finale, The Lions stood firm through five minutes of nerve-wracking injury-time.
Boro thought they had forced a dramatic leveller right at the death when Miller’s cross was stabbed home by McDonald, but the linesman's flag shot up immediately and just a few seconds later the final whistle blew - a magical sound to the 215 travelling fans and those watching on TV in the warmth of their frontroom!
Middlesbrough: Steele, Friend, Bikey, McManus, Dyer (Carayol, 46mins), McEachran, Leadbitter, Bailey, Main, Emnes (McDonald), Williams (Miller). Subs not used: Ripley, Ledesma, Hines, Smallwood.
Scorer: Main (75mins)
Millwall: Forde, Dunne, Beevers, Abdou, Shittu, Osborne, Woolford, Wright, Marquis, Afobe, Henry. Subs not used: M.Taylor, A.Smith, Trotter, Keogh, Malone, Tyson, N'Guessan.
Bookings: Osborne, Wright, Forde
Scorers: Friend (25mins og), Woolford (40mins)