CURTIS MAIN'S third goal in three starts since returning from injury was not enough to prevent Middlesbrough from slipping out of the Championship's play-off zone.
For the first time in four months Boro – who were top in November - are outside the top six after Millwall left the Riverside Stadium with all three points after first half frustration.
An own goal from George Friend and a first in Millwall colours for Martyn Woolford did the damage in a first half to Boro who lost for the seventh time in nine matches since the turn of the year.
Main's goal did find the net to make the last 15 minutes interesting, but Mowbray was left to rue first half errors – and a couple of offside decisions.
Millwall took the lead when Nicky Bailey conceded a free-kick. James Henry's free-kick was floated to the back post, where defender Karleigh Osborne was offside.
Without the whistle, Osborne's presence did enough to put off Friend whose diving header flew beyond his own goalkeeper, Jason Steele, to put Millwall ahead in the 25th minute.
And 14 minutes later Steele Millwall added a second. After Steele had denied Benik Afobe, who had cruised between Andre Bikey and Friend, Woolford reacted ahead of Bailey to turn the rebound over the line.
Middlesbrough, who introduced Mustapha Carayol, Ishmael Miller and Scott McDonald in the second half, did improve and got one back when Main pounced in the six-yard box to turn in Friend's run and cross in the 75th minute.
And when McDonald flicked Carayol's cross was flicked over the line in stoppage time Middlesbrough thought they had equalised, only for McDonald to have been ruled offside.