The 28-year-old former Leeds and Coventry player has signed a one year deal and will make his debut against Wigan on Saturday at The Den.
With Mark McCammon ruled out for four months and Richard Sadlier yet to return to full training, McGhee was desperate to sign another forward before the start of the season and had to play Paul Ifill up front during the pre-season campaign as an emergency striker.
Speaking about Whelan, the Millwall manager commented "In the last couple of years he's not quite been the player he was and I'm hoping that we can galvanise him once more. He's got a year's contract so he has something to aim for if he wants to earn a longer deal."
"We're a developing team and an ambitious team, and I think that Noel can play a big part for us this season. He's a Premier League player and he can make a real impact in the First Division"
But that may not be the end of transfer activity at The Den this week as McGhee also revealed that The Lions have bid for a second striker with the intention of improving the quality and depth of the existing squad.