Two goals from Duncan Watmore either side of a Marcus Tavernier strike inside 20 gave the Teessiders all three points after back-to-back defeats at Stoke and Preston.
Our Boro writer Joe Nicholson has dished out his marks from the match – scroll down and flick through the pages of our picture gallery to see how each player fared for the Teessiders:
9. Marcus Tavernier - 8
Rotated positions with Watmore and was a handful for Millwall’s slow defenders. Cut in from the left to score Boro’s second. 8 Photo: George Wood
10. Marvin Johnson - 8
Was a constant threat down the left. Assisted the opener and looked to cross the ball early. 8 Photo: George Wood
11. Chuba Akpom - 7
Regularly looked to run in behind Millwall’s defence and made chances for himself. Wasn’t able to register the goal he craved. 7 Photo: Getty Images
12. Djed Spence - 6
(for Watmore, 71) Saw a late shot blocked after moving to the right flank. 6 Photo: Getty Images