The former Republic of Ireland international, who as chairman put together the Drumaville consortium which backed the Black Cats return to the Barclays Premier League, and then persuaded American tycoon Ellis Short to take over, today announced he is to end his involvement as director of international development to concentrate on his family and business interests.
Quinn said: "I have had the most amazing six years and it gives me a huge sense of pride to see where Sunderland is today."
He added: "Sunderland is in an incredibly fortunate position to have a hugely talented team of people leading the football club right now.
"Everything is in place for Sunderland to really make a statement, which was always my aim.
"From a football perspective, securing the services of Martin O'Neill has been an absolute highlight.
"In Martin, we have a man that understands the region, the club and the unique place it holds in the lives of its supporters, and I am confident that he will harness the immense passion that exists here and take us to new heights.
"As for the great Sunderland fans, I would like to thank them for the trust they placed in me.
"Back in 2006, I had no business experience and little formal education, and yet the fans backed me and believed in what I was trying to do.
"They came back to the club in their tens of thousands and the faith they showed in me was humbling."
Quinn stepped in after Sunderland's humiliating return to the Football League at a time when they had managed to collect just 15 top-flight points during the 2005-06 campaign.
That return had cost manager Mick McCarthy his job in the March and with Kevin Ball having ended the season as caretaker, Quinn and his newly-installed Irish consortium sought the man they believed the club needed to move forward.
The former striker, who had represented Sunderland with such distinction under Peter Reid, endured a brief and unsuccessful spell in the hot-seat himself before making a major splash by appointing Roy Keane, and although his reign lasted just two-and-a-half years, the recovery he inspired has seen the Black Cats retain their Premier League status for five seasons.
Quinn said: "I am now looking forward to spending more time with my family and exploring business interests away from football.
"One thing is for sure though, I'll still have that knot in my stomach before kick-off and the feeling of pure elation at final whistle if the result has gone our way - that will never leave me."
Short saluted Quinn's contribution to the club during his tenure, and vowed to carry on his work.
He said: "Niall Quinn is and always will be a Sunderland legend. His vision brought me into the club and that vision still inspires what we do.
"He has been a trusted friend and advisor to me throughout our time together at Sunderland, and whilst I'm sad about his departure, I respect his view that his work is done. My job is to carry on that work."