John G. Berylson, who is currently the chairman of Millwall Holding PLC, added the chairmanship of the football club to his portfolio with former incumbent Stewart Till stepping down to become vice-chair.
The switch is one that, according to the American investor, has always been on the cards since he arrived at the Club.
"It has always been my intention to assume chairmanship of the football club once I became more knowledgable," Beryslon told the club's official website. "All major decisions concerning the football club require my agreement as the main investor and it is important that we have clear lines of accountability and responsibility.
"I'm grateful to Stewart and my other board colleagues for all their help and support and I very much look forward to working with them in the future."
Till added: "This new arrangement is the right one for the club going forward and I'm delighted to continue to be involved as we all work together for the good of Millwall."