Millwall have announced that their plc chairman John G Berylson has also taken over the role of chairman of the football club from Stewart Till.
Berylson's consortium invested in Millwall in March 2007
Berylson heads the US-based consortium Chestnut Hill Ventures, which invested in the club in March this year.
Till will remain on the board and become vice-chairman.
"It has always been my intention to assume chairmanship of the football club once I became more knowledgeable," Berylson told the Lions' website.
"All major decisions concerning the football club require my agreement as the main investor, and it is important we have clear lines of accountability and responsibility."
The Lions are stil searching for a new manager, following the recent sacking of Willie Donachie that has left Richard Shaw and Colin West in caretaker charge.
League One rivals Yeovil have already turned down an approach by Millwall to speak to their manager Russell Slade, while Manchester City reserve coach Kenny Jackett has ruled himself out of the running.