Holloway, who has guided both Blackpool and Crystal Palace to the Premier League in recent seasons, believes that this tops all his previous endeavours.
"This is the greatest achievement of my managerial career," the 51-year-old told reporters following the game. "We had to keep our mind in every game and not become obsessed with results.
"I'm proud for my players. We've got a proper Millwall team now. I'm so chuffed for everybody associated with this great football club. Millwall is real and I'm proud to be here."
Millwall finish the season in 19th place in the Championship following the win.