Full-time score from The Den
Two goals from Andy Keogh coupled with a Liam Trotter cracker secured Millwall's first home win of the Championship campaign.
Skipper Trotter put The Lions into a 25th-minute lead with a thumping 20-yarder before Keogh scored his brace on 34 and 86 minutes either side of a Marvin Emnes' penalty for the visitors (43mins).
This proved to be a thoroughly deserved success for Kenny Jackett's side, who temporarily moved up to seventh in the table with this three-point haul.
Boss Jackett named an unchanged team for the second successive game, keeping faith with the side that lost in such heart-breaking circumstances at Sheffield Wednesday last weekend.
And his faith paid off as The Lions at times produced some super free-flowing football.
Credit to Tony Mowbray's Boro outfit too, who played their part in an open and entertaining encounter.
In the opening seven minutes Millwall were twice caught out by the narrowest of offside decisions as Trotter and then Darius Henderson were stopped in their tracks by tight calls from the linseman.
The Lions could have opened the scoring with nine minutes on the clock when the irrepressible James Henry's corner was knocked on by Karleigh Osborne to Keogh, and his fierce snap shot was held by the perfectly placed Boro keeper Jason Steele.
Keogh went even closer on 18 minutes, meeting Henry's deep delivery from the right with a powerful downward header that was brilliantly parried by Steele, and then seeing his second attempt smash against the woodwork before the danger was cleared.
Boro had created a couple of chances of their own in this period, both falling to new signing Ishmael Miller who volleyed over and also fired a firm shot straight into the hands of Maik Taylor in the home goal.
However the deadlock was finally broken midway through the half - and what a pearler it was too as Trotter sweetly struck a bouncing ball on the half volley into the top corner with an effort that gave Steele no chance.
Millwall doubled their tally barely 10 minutes later when Keogh slammed home the rebound after Henry's initial shot had hit the post.
The Lions were by now looking comfortable, only for Boro to be handed a lifeline a couple of minutes before the break. Jimmy Abdou's rash challenge on George Friend saw ref Fred Graham point to spot and brandish a yellow card to the midfielder.
Up stepped Emnes, who made no mistake with the penalty to reduce the arrears.
Striker Henderson was replaced by Shaun Batt at the start of the second half as Millwall looked to press Boro's back-line, and the visitors were at sixes and sevens soon after the restart; Henry's free-kick caused chaos in the visiting defence but neither Keogh, Osborne or Trotter could connect as the ball ricocheted in the danger zone.
Henry was involved again, this time at the other end of the pitch producing some fantastic defensive work - and not for the first time either - blocking Friend's cross after tracking back from deep in his own half.
Boro continued to push for an equaliser but Millwall refused to sit back and in a real end-to-end game produced further chances of their own, the best of which saw Scott Malone's effort dribble agonisingly wide following a slick counter-attack involving Henry and Keogh.
Malone, who along with fellow full-back Alan Dunne got forward well to support the attack, had a hand in the third, killer goal five minutes from time with a powerful run and cross down the left that picked out Keogh, whose clever flick bounced in off the far post.
Boro could have set up a grand-stand finale a couple of minutes later, but Emnes smashed the ball high over the bar from just five yards and with that went any hope they had of clawing their way back.
Millwall: Trotter (25mins), Keogh (34, 85mins)Scorers:
: Henry (32mins), Abdou (43mins), Lowry (75mins) Bookings
M.Taylor, Dunne, Malone, Abdou, Lowry, Osborne, Trotter, Wright, Henderson (Batt, 46mins), Keogh, Henry (Feeney, 82mins). Subs not used: Forde, Shittu, Racon, Marquis, C.Taylor.
Middlesbrough: Emnes (43mins, pen)Scorer:
Steele (32mins), Haroun (33mins), McEachran (50mins) Bookings:
Steele, Emnes, Woodgates, Zemmama (Ledesma, 54mins), Hines, Leadbitter, Miller (Williams, 73mins), Haroun (Smallwood, 62mins), McEachran, Friend, Hoyte. Subs not used: Ripley, McManus, Reach, Arca.
Ref: 9,576 (away 768)Att: