The Irishman has now decided to concentrate on his family and business interests outside of football.
Speaking about his decision, Niall said: 'I've had the most amazing six years and it gives me a huge sense of pride to see where Sunderland is today.
'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. 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.
'I must make special mention of the Drumaville consortium, without whom the story would not have unfolded. They will always be fondly remembered on Wearside and I will always be grateful for their input. Later on Ellis came in - and he believed too.
'As the guardian of Sunderland in recent years, he has proven himself to be astute, wholeheartedly committed and above all passionate about this football club. He has truly bought into the ethos of what Sunderland is all about and this is so much more than a business to him.
'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 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.'
Chairman Ellis Short 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.
'His hard work laid down the foundations that we are now building on and the vision and drive he showed both in those early days and indeed in recent times as he focused on our international development, particularly in Africa, should never be underestimated.
'We fully understand his desire to take a back seat now and to concentrate on his own life and interests away from football and he deserves that opportunity.
I would like to express my own personal thanks to Niall for his support and he leaves with our very best wishes and immense gratitude for all he has achieved.
'He will always have a unique and special place in our history and indeed in the hearts of every Sunderland fan.'
Manager Martin O'Neill said: 'Niall has been a truly iconic figure at Sunderland, both as a player and in his time leading the club from the top. 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.'
Chief executive Margaret Byrne added: 'Niall has overseen a period of huge transition for the football club with his own unique style of leadership.
His passion and enthusiasm are quite simply infectious and his desire to see Sunderland compete with the country's top clubs is something that everyone here is committed to taking forward.
'I echo the sentiments Ellis has expressed and would like to place on record my own thanks to Niall, from both a personal and professional perspective, for the support and guidance he has afforded me in the five years I have been at the club.
He leaves behind a wonderful legacy and one which will stand our club in great stead for the future.'