Former striker quits his role after six years at the helm to spend more time with his family in Ireland, but thanks fans for an "amazing" time with the club
The Irishman, who scored the first goal at the Stadium of Light, stepped down from his position as chairman toward the end of 2011 but will now leave the club permanently.
The former striker has been focussing on increasing the club's international profile recently but wishes to concentrate on spending more time with his family in Ireland.
"I've had the most amazing six years and it gives me a huge sense of pride to see where Sunderland is today," Quinn told the club's official website.
"Sunderland is in an incredibly fortunate position to have a highly talented team of people leading the football club right now.
"There is a tremendous amount of enery about the place and the board and the executive management team are a dynamic and committed group, who hold the best interests of the club at the core of all they do.
"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.
"Sunderland supporters can be confident that their club is in very safe hands, as am I. I am now looking forward to spending more time with my family and exploring business interests away from football.
"One thing is sure though, I'll still have that knot in my stomach before kick-off and the feeling of pure elation at the final whistle if the result has gone our way - that will never leave me."
Quinn led an Irish consortium in buying Sunderland in 2006 and spent a short spell as both manager and chairman before appointing Roy Keane.
His role changed after owner Ellis Short purchased the club in 2011 and the American now occupies the role of chairman in Quinn's place.
"Niall Quin is and always will be a Sunderland legend," Short told the club's website. "I respect his view that 'his work is done'. My job is to carry on that work.
"He has been a wonderful servant to Sunderland and his determination to see the club grow has been inspirational. He will always have a unique and special place in our history and indeed the hearts of every Sunderland fan."
"Niall has been a truly iconic figure at Sunderland, both as a player and in his time leading the club from the top," added manager Martin O'Neill.
"Like everyone, I couldn't be more disappointed that he has decided to step down but of course I respect and understand his decision. He has been the heartbeat of the football club for so long and his legacy is immeasurable. To me he is 'Mr. Sunderland' - and always will be."