Quinn stepped down as chairman last October to be replaced by Ellis Short and had been working in an international development role in trying to raise the club's profile abroad.
The 45-year-old headed the Drumaville Consortium in June 2006 to buy Sunderland with the former Sunderland striker then becoming chairman.
Quinn, who retired from playing in 2003 at the Stadium of Light, believes he has left the club in good hands under Short and manager Martin O'Neill as he plans to return to his native Republic of Ireland to explore other business opportunities away from football.
"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 hugely talented team of people leading the football club right now.
"There is a tremendous amount of energy 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."
Chairman Short paid tribute to Quinn and has vowed to carry on the work done by the Irishman on Wearside.
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.
"He has been a wonderful servant to Sunderland and his determination to see the club grow has been inspirational."
O'Neill also praised Quinn's impact at Sunderland and admitted he played a big part in his decision to move to the Stadium of Light.
"Niall has been a truly iconic figure at Sunderland, both as a player and in his time leading the club from the top," noted O'Neill.
"His vision and drive, alongside that of Ellis, played a significant part in me coming here.
"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."