The Scotsman is regarded as the most successful manager in British football following a career that has accumulated 13 Premier League titles, five FA Cups, two Champions League trophies and a host of silverware, which has cemented United as one of the elite clubs in the game.
While there had been no suggestions from the 71-year-old that he intended to step down from his role by the end of the season, Ferguson has since confirmed that it is the "right time" to go.
Sports Mole rounded up the latest developments regarding Ferguson and United's future over the course of the day below.
"It was important to me to leave an organisation in the strongest possible shape and I believe I have done so.
"The quality of this league winning squad, and the balance of ages within it, bodes well for continued success at the highest level whilst the structure of the youth set-up will ensure that the long-term future of the club remains a bright one."
Premier League: 1993, 1994, 1996, 1997, 1999, 2000, 2001, 2003, 2007, 2008, 2009, 2011, 2013
FA Cup: 1990, 1994, 1996, 1999, 2004
League Cup: 1992, 2006, 2009, 2010
Uefa Champions League: 1999, 2008
Fifa Club World Cup: 2008
Uefa Super Cup: 1992
Uefa Cup Winners Cup: 1991
Inter-Continental Cup: 1999
FA Charity/Community Shield: 1990 (shared), 1993, 1994, 1996, 1997, 2003, 2007, 2008, 2010, 2011
He wrote: "I don’t have any plans at the moment to walk away from what I believe will be something special and worth being around to see.
"It’s always difficult in football to be absolutely sure of the future because the game has a habit of tripping you up, but I don’t live in a fantasy world and believe we have every reason to feel confident about the future of Manchester United. My view stems not from the euphoria of winning back the title we lost last season, but on the way we did it and the make-up of our playing personnel."
The defender tweeted: "Sir Alex, one of the best managers of all time. After 26 years of success in the game, we all owe him a tribute."
Ince told Sky Sports News: "I'm totally shocked. What's he's done in unbelievable. You'll never see anyone of his kind again.
"Two weeks ago he was talking about staying on for another two years, so it's a massive, massive shock. The way he treated me was like a son."
"What you have is a large proportion of the crowd who only know Sir Alex Ferguson as the manager," Stirling told BBC News.
"They don't remember the past managers. It is going to be an interesting time for these supporters. Older Reds like myself remember the transitional periods and it can be dangerous times."
Greg O'Keeffe of the Liverpool Echo tweeted: "EFC say they're unaware of anything imminent in terms of a decision from David Moyes on his future today. #manutd #alexferguson"
He wrote on Twitter: "Manchester United source tells me Sir Alex Ferguson's successor will be announced within days but has to resign from current role first...
"My #MUFC source also tells me club's board has been in talks with Fergie's successor for some time. Rules out neither Moyes nor Mourinho."
The striker wrote on Twitter: "It's just not sinking in! Manchester United with no Sir Alex just doesn't feel right. What a man and great that it has ended on his terms. What a privilege to have played under arguably the best manager the world has ever seen.
"His record will surely never be eclipsed. Simply astonishing to read through his managerial record of achievements."
Owen tweeted: "What an act to follow. I guess only Mourinho would have the confidence to strut through the door. If it were me, I'd go for Moyes."
He wrote on Twitter: "Just heard Sir Alex Ferguson is retiring at end of season. His achievements in the game place him without doubt as one of the 'greats'. Was honour to present Sir Alex with award at 2011 Ballon D'Or. Will his longevity at the top ever be repeated?"
"The game of football will be a lot poorer place without him. He has been quite simply the best. He followed in Sir Matt Busby's footsteps and even surpassed him. He will be also be the toughest act to follow."
He's said: "Disappointed, shocked, sad. Didn't think THAT day would be today."
He wrote on Twitter: "Sir Alex Ferguson’s achievement at #MUFC has been exceptional. Hopefully his retirement will make life a little easier for my team #AVFC"
"All at Chelsea would like to wish Sir Alex Ferguson the very best in his retirement. He has been a fantastic adversary over 26 years."
West Ham: "Everyone at #WHUFC would like to wish Sir Alex Ferguson a very happy retirement following 26 successful years as Manchester United manager."
Arsenal: "Everyone at Arsenal wishes Sir Alex Ferguson health and happiness following the announcement of his retirement as Manchester United manager."
He told MUTV: "It really has caught me by surprise. My phone kept ringing all morning and I thought it must be something serious. When the news comes through, it's about the manager, which is serious if you're a Manchester United supporter, everybody will be a little bit concerned. Can the next guy coming through the door do what Sir Alex has done? That's going to be tough."
FA Chairman David Bernstein has reflected on Sir Alex's 'extraordinary' achievements.
"Sir Alex Ferguson’s achievements are truly remarkable – he is genuinely one of the greatest managers of all time and certainly of the modern era.
"His contribution to English football has been outstanding in every regard and, in The FA’s 150th year, it is something that should be celebrated. On behalf of The FA I’d like to wish him a happy and healthy retirement."
He said: "[Ferguson] called us into his office and said what decision he took. Its always been on the cards, I think the manager kept his cards close to his chest. I think he felt the time was right. He's been absolutely fantastic for this club and I hope the fans will give whoever comes in the same support that Ferguson got."
"It takes a leap of imagination to believe Manchester United do not already have Sir Alex Ferguson's chosen successor in place. The choice is an interesting one. Do they go for the charisma, successes - but essentially quick hits - of Jose Mourinho or the stability they know Everton's David Moyes would give them? In trophy terms there is only one choice but United will want to consider the wider context."
"The Premier League has had the privilege to witness many great players, managers and teams," BBC Sport quotes Scudamore as saying. "No one has made as great a contribution to the Premier League than Sir Alex Ferguson.
"His drive, ambition, skill, passion and vision have not only shaped Manchester United, but in many ways the game of football as we now know it. Sir Alex and his teams during the Premier League era have set the pace others have had to follow; 13 titles out of 21 seasons is a phenomenal achievement."
They are: David De Gea, Rafael, Phil Jones, Jonny Evans, Anderson, Chris Smalling, Javier Hernandez, Nani, Danny Welbeck, Tom Cleverley, Nick Powell, Shinji Kagawa, Alex Buttner, Ben Amos.
On Monday, Ferguson will address the fans alongside his players at a send-off event at Old Trafford at 6pm, which will then be followed by the bus parade at 6.30pm. The parade will leave Old Trafford and drive through the streets of Manchester before concluding the route at Albert Square.
O'Keeffe wrote on Twitter: "Increasingly hearing that David Moyes WILL leave #EFC to succeed Alex Ferguson at #MUFC in the summer."
"His dedication, his attention to detail and his unique eye for talent, as both the manager of Manchester United and Aberdeen, has brought rich rewards over a 30-year period. His CV is almost unique in a results-based profession that normally focuses on short-term solutions rather than long-term vision."
"It would not surprise me," Bruce told Sky Sports News. "He's a very talented manager who has been at Everton for more than 10 years. He's not had huge finances but he has still managed to put an Everton team together that will comfortably finish in the top 10. He has been in the job a long time and he has all the credentials to be a top manager for Manchester United."
Gill, who will step down from his role at the end of the season, has claimed that the club have been "preparing" for the inevitable.
"It is a new era and I look forward to watching a successful Manchester United team on the pitch," Gill told the club's official channel. "It is a momentous day for the club, the fans and the whole football world.
"We knew this day would come and we have been preparing for this. The board will take council from Sir Alex and Sir Bobby [Charlton] on new appointment. Working with Sir Alex has been the highlight of my career."
He wrote: "Fergie retires. What an act to follow. All the best to whoever picks up the gauntlet. Looks like Moyes. Big loss for EFC but great opportunity."
He told MUTV: "We have to look forward now and I think any manager coming in will inherit a great squad and great staff around him. It is a dream job. I won't speculate on the announcement but I think we will move relatively quickly.
"In Sir Matt Busby and Sir Alex we got in managers who got involved in all aspects of the club. That degree of loyalty is crucial to the success of Manchester United. [The next manager] needs the required football experience in terms of domestic and European requirements, so it is a small pool."
Sky News journalist Paul Kelso has tweeted: "Borussia Dortmund say Jurgen Klopp will not be succeeding Sir Alex Ferguson at #MUFC. 'He has always said he will stay until 2016'"
"I think it's clear he was going to announce this at the end of the season, but whispers have got out somewhere, so he probably had to make it official right now."
"For me, he is the greatest club manager that has ever been produced, certainly in this country, if not Europe. However many managers there were before him, the huge task he had was to achieve what Sir Matt Busby had done before him. For him to have done that and more, to have won the treble for Manchester United, and the joy and satisfaction of doing that gave was fantastic."
Journalist Sam Wallace wrote on Twitter: "#MUFC sources say Mourinho was never in the running to succeed Ferguson. As far as they're concerned his deal with #CFC is done & dusted ..."
Meanwhile, The Times are reporting that David Moyes will be appointed as Ferguson's successor within the next 24 hours.
"No one will be able to match his achievements, his dedication, his support for colleagues in need and his team building know-how. This is a sad day for English football. I personally wish him a happy and successful retirement."
Here's a reminder of the ceremony at Old Trafford:
He wrote on Twitter: "Stand up, put your hands together and give him a big round of applause! Sir Alex Ferguson is retiring. Congrats for an amazing career!"
"He is such a fantastic manager," the Daily Mail quotes Charlton as saying. "Everything he has done has been fantastic. He is a sensational person in every form and I am really delighted for him.
"I am a director but I hardly do anything because we are winning all the time and it is all down to Sir Alex Ferguson. He would get up in the middle of the night and travel 300 miles if he thought there was a school boy that he could sign. He loves the game and we at the club have had nothing to do really."
"It is unbelievable to change around probably four different squads and have the success he has had and rebuild a team like that - that is really good management and it is a difficult management skill to achieve."
"He will always be the boss to me, you don't think of him as Sir Alex it has always been the boss."
The Australian wrote on Twitter: "Sad day for football. The Great Man retires. One of the greatest managers ever. SAF is 1 of the most influential people that I have met #KING"
"He is just a couple of boxes down from me so I am going to hopefully have a drink with him later," Owen is quotes as saying. "He is arguably the greatest manager to have ever lived and to say I played under him for three years is a proud thing to say."