Former chairman Quinn to leave Sunderland
The former Ireland international striker, who stepped down as chairman last October, told the club website that he would return to Ireland to focus on his family and business interests outside soccer.
Quinn had been carrying out an international development role at the north-east club, who beat Arsenal 2-0 on Saturday to reach the quarter-finals of the FA Cup and are ninth in the league.
"I've had the most amazing six years and it gives me a huge sense of pride to see where Sunderland is today," said Quinn. "Everything is in place for Sunderland to really make a statement, which was always my aim."
O'Neill, who joined Sunderland in December when the club were 16th in the league, said he was disappointed by Quinn's decision but respected it.
"He has been the heartbeat of the football club for so long and his legacy is immeasurable," the former Northern Ireland captain said. "To me he is 'Mr Sunderland' and always will be."