Having taken over as boss in 1986, Ferguson set about creating a dynasty of his own at the club. He built several great teams and won an abundance of medals, totaling 38 trophies overall.
He will step down after the final game of this Premier League season, which will be his 1,500th match in charge of United.
It has been a long and colourful career; here are the very best and worst parts of his long reign.
Alex Ferguson was chosen to take over at Manchester United in November 1986.
United were 19th in the old Division One, and Ferguson was charged with taking the club to safety and turning around their fortunes. He managed to lead them to 11th place by the end of the season, with the club having won, lost and drawn 14 matches.
It was the start of a long journey, one which had started at the very bottom of the top league.
In his first full campaign in charge, Ferguson led United to a second-place finish in the First Division, losing just one game at home all season. The next season, though, 1988-89, saw United end back in 11th once again, as inconsistencies plagued Ferguson's early team.
In 1989-90, United had another difficult start, ending the decade in 15th position, just two points outside the relegation zone.
Fans had had enough, and there were plenty calling for his head, as United finished the season in 13th.
But that season wasn't all doom and gloom—far from it, as United won silverware for the first time that campaign under the guidance of Alex Ferguson.
An FA Cup win, the first of five that Ferguson won during his time at Old Trafford, came in May against Crystal Palace, as United won 1-0 in a replay.
It was the first of 38 pieces of silverware that Ferguson would accumulate in his time as United boss.
United got themselves back into the top six during 1990-91, but Ferguson also suffered his heaviest defeat at the hands of one his his biggest rivals, Liverpool, during that campaign.
The Red Devils were soundly beaten by the Reds by a 4-0 scoreline. Liverpool were inspired by Peter Beardsley, who scored a hat-trick, and United were left to focus their attentions elsewhere that season.
While domestically they were still well off the pace, Manchester United won a trophy in Europe, as Alex Ferguson guided them to glory in the UEFA European Cup Winners' Cup.
United became the first English side to win a European competition since the mid-1980s when they beat Barcelona in the final, winning 2-1 thanks to a Mark Hughes brace in the second half. United's entire starting XI and substitutes for that final were comprised of players from England, Ireland, Wales and Scotland.
1991-92 saw Manchester United finish as runners-up to Leeds United in what was the last season in the old Division One format, but when the Premier League era began, so did the real dominance of United under Alex Ferguson.
United won the league by 10 points over Aston Villa, their first league title since 1967. The midseason arrival of Eric Cantona proved pivotal to United's form in winning the championship.
One year later, United managed to retain their league trophy, finishing eight points ahead of Blackburn Rovers. It turned into a double celebration, though, as they also tasted glory in the FA Cup, winning 4-0 against Chelsea in the final.
It wasn't quite a treble, though, as they lost to Aston Villa in the League Cup final.
Roy Keane was the big signing of the summer, joining from Nottingham Forest and starting his own long and impressive career at Old Trafford in the midfield.
A season of woe was to follow, as Manchester United saw forward Eric Cantona banned for eight months for an attack on a Crystal Palace supporter, at whom he jumped over the hoardings to kick and aim punches after being insulted.
On the pitch it was equally difficult, as United missed out on the title on the final day of the season. They could only manage a draw at West Ham when a win would have snatched them top spot, with leaders Blackburn losing to Liverpool.
The FA Cup final could have provided some solace—but United lost that, too, defeated 1-0 by Everton.
United sold a number of important senior players in the summer prior to the 1995-96 season, including Paul Ince, Andrei Kanchelskis and Mark Hughes.
Ferguson didn't bring in any big-name signings, leading to plenty of people wondering how well United would fare that season...but they reckoned without the progression into the first team of a certain group of youngsters.
Namely: Nicky Butt, Gary and Phil Neville, David Beckham and Paul Scholes. They remain arguably the best crop of youngsters to have come out of a single club within the same couple of seasons of each other.
United won the league again that season, and again in '96-'97.
1997-98 wasn't such a good year for United.
Despite leading the Premier League table at the beginning of March, they slipped on the home straight, winning just five of their last 10 matches and finishing the season in second, a point behind Arsenal.
A 1-0 defeat at home to the Gunners in mid-March proved pivotal to the destination of the title, while United also flopped in Europe, losing to Monaco in the Champions League quarterfinals.
However, United came back with a bang the next season.
Ferguson and his team won the Premier League by one point ahead of Arsenal, beat Newcastle United 2-0 in the FA Cup final, and then turned the Champions League final around in injury time, coming from a goal down to win 2-1 thanks to goals from Teddy Sheringham and Ole Gunnar Solskjær.
It was one of the most memorable comebacks ever in European football.
Fergie's treble was perhaps his finest moment ever as boss of United.
United followed up the treble season by going on to win the Premier League again in 2000, and then again in 2001.
That made them the first side to win three league titles in a row since the change from the old Division One format to the new Premier League.
Quarterfinal defeats in both years in the Champions League meant that their European glory was not repeated, however.
Ferguson would have to wait almost a decade for his second title in that department.
2001-02 saw United finish third, their lowest Premier League placing for some time—but of most irritation to Ferguson would have been that Arsenal managed to clinch the title by beating United at Old Trafford.
A poor run of form from October to December saw United win just once in seven league games, losing five of them to rivals Liverpool, Arsenal and Chelsea, while only a last-minute equaliser from Solskjær prevented another defeat to Leeds in the middle of that run.
Ferguson was due to retire at the end of that season; needless to say, he didn't.
Obviously determined to come back stronger than ever, United were seven points off first place on Boxing Day, having lost to Middlesbrough, but they then embarked on a magnificent run of form, winning 15 and drawing three of their last 18 games of the season.
En route to that title-winning end to the season, Ferguson masterminded his biggest win over Liverpool, winning 4-0 at Old Trafford, and also guided his team to a 6-2 spanking of Newcastle United, who finished third.
United did suffer defeat in the League Cup final, however, losing 2-0 to Liverpool.
The next season proved an up-and-down one for United and for Ferguson, as they began a run of three years without winning the league title for the first time since their first championship win under the Scotsman.
That season also saw Rio Ferdinand banned for missing a drug test, and saw Ferguson bested for the first time by Jose Mourinho, then Porto manager, as his side knocked out Manchester United en route to winning the Champions League.
There was recompense in the form of a fifth FA Cup win, though, with United easily beating Millwall 3-0 in the final. It would be the last time Ferguson lifted that particular trophy, as he faced a decade of frustration in the competition thereafter.
A League Cup trophy was the scant consolation for United in 2005-06, as they ended runners-up to Chelsea in the Premier League and suffered a humiliating Group Stage exit from the Champions League.
In fact, it was to be their worst Champions League performance under Alex Ferguson, as they finished bottom of the four-team group, winning just one of their six matches.
A win in the last group match at Benfica would have seen United go through in second, but instead they lost 2-1 and finished last.
A single moment in the 2006-07 season was one of the most disappointing of all of Alex Ferguson's long tenure, as his Manchester United side were beaten 1-0 by lowly Southend in the League Cup.
The defeat came just one day after Ferguson had celebrated 20 years as Manchester United boss, far from a fitting way to mark the occasion.
United's team that day included such stars as Wayne Rooney, Cristiano Ronaldo, Darren Fletcher and Gabriel Heinze.
The trophy Ferguson craved finally came around once more in 2007-08, as United lifted the European Cup for the second time under his leadership.
This time, they defeated fellow Premier League side Chelsea in the final, winning 6-5 on penalties after a 1-1 draw in normal time in Moscow, Russia.
United also won the Premier League ahead of Chelsea, with a two-point margin of victory.
Manchester United and Alex Ferguson saw their old title rivalry with Liverpool rekindled for the 2008-09 season, and in early March it seemed as though it might be the Merseyside team who were in the right form heading into the final straight, as they blitzed United 4-1 at Old Trafford.
It was Ferguson's second-heaviest defeat against his longtime rivals since taking charge of United, but there was to be a far better ending to the season for the Red Devils, as they went on to win eight of their final nine games to clinch the title ahead of Liverpool by four points.
It also ensured a second three-in-a-row party for United, having won titles in '07, '08 and now '09. They also wrapped up a League Cup title this season.
There was just one more crushing blow for United to come that season, though.
Despite making it to another European Cup final, they found their opponents Barcelona one step ahead and were defeated by a 2-0 scoreline.
Like many teams before them, and many more yet to come, they found Leo Messi too hot to handle.
Despite losing out on the Premier League title the following season to Chelsea, United and Ferguson were back in 2010-11 to clinch yet another championship—and this one was the club's 19th of their history, making them the most successful top-flight English side in history.
United didn't lose a league game until February that season and finished nine points ahead of their nearest rivals.
Deja vu for United, as despite the league title they were yet again bested in the European Cup final by Barcelona.
Ferguson had hoped to have learned significantly from two years earlier, and went for a more offensive approach to the game, but Barcelona were irresistible and dismantled his team, winning 3-1 this time.
In contrast to his first European final back in 1991, also against Barcelona, this team contained four Home Nations-born players, an indication of both the growth of the game and the globalisation of Manchester United under Ferguson's reign.
As we come closer to the end of Ferguson's time in charge, it is fitting to include two of the most memorable matches over which he has presided.
First up, the 2011-12 season saw an enthralling match at Old Trafford, when Ferguson outwitted, out-footballed and outfought his adversary Arsene Wenger, with Manchester United triumphing by a whopping 8-2 scoreline over Arsenal.
Wayne Rooney (3), Ashley Young (2), Nani, Danny Welbeck and Park Ji-Sung all got on the scoresheet for United.
Though the game came extremely early in the season, the scoreline appropriately left United top of the Premier League, and Arsenal just one place above the relegation zone.
It was just two months from that victory in August to Ferguson's most brutal day in the dugout, United's heaviest defeat to neighbourhood rivals Manchester City since 1955.
Old Trafford was once more the scene of the show, as City ran out incredible 6-1 victors on their way to winning their first Premier League title.
Darren Fletcher scored a late goal for United, but City were already three goals up by that point and added a further three goals in injury time to inflict a memorable day, for all the wrong reasons, upon Alex Ferguson.
And so to the most recent of days, as Manchester United wrapped up their 20th league title, and their 13th under Alex Ferguson, just a couple of weeks ago.
Still with two more matches to play this season, Ferguson has been able to make up his mind about retiring in advance, thanks to the title being confirmed early.
They lead Manchester City by 10 points, and Ferguson will leave United on top of the English football pyramid.
It will now be for his successor to work out exactly how to keep them there—but with the weight of 26 years of experience and 38 trophies behind him, whoever takes over as United boss will have a hugely difficult task to replace Ferguson, both in terms of his personality and his incredible success.