The visitors remained top of the Sky Bet Championship despite their first defeat of the season, but they were very much second best at The Den, where the Lions missed plenty of chances before finally scoring.
Harris said: “We’ve played in a lot of games this year where we’ve been as good as that and not got the points, so it’s very pleasing to play that well and get the three points.
“We knew it was going to be tough because Leeds have scored the most goals and conceded the least. It makes them the best team in the league.
“We worked very hard on Thursday and Friday to make sure the team’s got a game-plan to stop them playing, and the credit goes to the players for executing it.
“Not only were we the better team on the pitch, with the ball and without the ball, but you’ve got to remember that last Saturday at Wolves, almost an identical team played for 30 minutes with 10 men.
“Then on Tuesday night we played with 10 men for 60 minutes and I almost picked the same team again and I thought we outfought, outran, won every first ball and every second ball today.”
Millwall had an early goal disallowed when an offside Steve Morison followed up after Conor McLaughlin’s shot was spilled by Felix Wiedwald, who did well to deny Shaun Hutchinson early in the second half.
Jed Wallace, one of the game’s outstanding performers, than struck a post from the edge of the area before O’Brien broke through on 73 minutes by turning in a loose ball.
Leeds boss Thomas Christiansen said: “The ideas for this game were clear; we knew that it would be a direct play, an aggressive game, with fast transitions to attack, but we were not able to stop that.
“We had to stop it and go into the battle with the same intensity as Millwall did.
“I want to see a reaction after a defeat. We have to learn from our mistakes and bring it to the next game.
“I cannot complain (about) the players for anything because if we are in the first position, still now, it’s because of them, so I will not complain for one loss in a very bad game.
“We didn’t do the things right during the 90 minutes and everyone has the right to make mistakes.
“I’m of course not happy with the result, but we have to face the fact that we didn’t play a good game and Millwall deserved this win.”