Michael Gearon Jr., an Atlanta-based member of the ownership group, said Mullin's resignation Tuesday will set up a more efficient management of the teams.
Mullin had been president and chief executive officer of Atlanta Spirit, representing the ownership group for the Hawks, the Thrashers and Philips Arena, since 2004.
"As far as the transition goes, it's seamless," Gearon told The Associated Press. "There's no impact at all to the operations."
Gearon said the "mutual decision" for Mullin and chief financial officer Bill Duffy to resign "has nothing to do with the teams" but was a result of the ownership group's active role in the operations of the teams.
The move comes as the Thrashers (23-24) are preparing to host the NHL All-Star Game on Sunday.
"We've gotten to know the management team very well and we're very comfortable with them and we don't need somebody between them and us," Gearon said. "There's no negative feelings on either side."
Mullin said he's leaving to pursue other opportunities.
"I will always cherish my time with the Atlanta Spirit and I'm proud to have helped put into place what I believe will be one of the most successful organizations in professional sports," Mullin said.
"Both teams are in a position to be perennial playoff contenders and I will particularly look back with pride knowing that Philips Arena is now one of the top three revenue-generating arenas in the world," he said.
Executives who have reported directly to Mullin will now report to the owners, including Gearon, who will be the point man for the Hawks (17-20). Gearon already serves as the team's NBA governor, the member of the group who must sign off on all trades.
Gearon said Hawks general manager Billy Knight, Thrashers general manager and interim coach Don Waddell and Philips Arena president Bob Williams already report to owners. Executive vice president for sales and marketing Lou DePaoli and chief legal officer Scott Wilkinson also report to owners.
"Frankly, when we first hired Bernie there was an expectation everybody would report to Bernie and Bernie would report to the board," Gearon said. "It never turned out the way originally envisioned."