McLeish was told only three weeks ago that his job was safe at Birmingham, following the Blues' relegation from the Premier League, with the board at St Andrew's loyally standing by their manager after he had delivered the club its first silverware since 1963 and led them into Europe.
But the former Motherwell, Hibs, Rangers and Scotland manager dropped his bombshell yesterday afternoon, confirming his unexpected resignation in a note sent to acting chairman Peter Pannu.
Birmingham announced in a statement on their website: "Birmingham City Football Club can confirm that Alex McLeish has resigned as manager. Acting chairman Peter Pannu received an e-mail resignation from McLeish earlier today while away on business in Amsterdam.
"No further comment will be made at this moment in time and updates will be released in due course."
McLeish's departure came as the 52-year-old appeared to be rebuilding the squad at St Andrew's for next season's campaign in the npower Championship. In the past week he had added former Rangers winger Chris Burke and striker Marlon King to the Blues squad, and he looked set to stay and lead the pursuit of promotion.
Bookmakers were moved to immediately declare McLeish their hot favourite for the Villa job, vacated recently by Gerard Houllier, with some firms quoting him as low as 1-5. Villa owner Randy Lerner is said to be an admirer of McLeish and has spoken in glowing terms of the job the Scot has done operating on a tight budget with Birmingham.
However, being the favourite for the Villa job does not guarantee it will happen, and several managers have been in that position since Houllier's departure was announced on 1 June. It remained to be seen last night whether there was any correlation between his resignation and the nearby job vacancy.
Although McLeish led Birmingham to Carling Cup glory last season, beating Arsenal 2-1 in the final, his side's form in the Premier League dipped after their Wembley success, culminating in relegation on a dramatic final day of the campaign.
If the timing of McLeish's resignation came as a surprise, it would be a stunning turn of events if he was handed the top job at Villa. McLeish was appointed by Birmingham in late 2007, after achieving some eye-catching results with the Scotland national team, but was unable to save the club from relegation at the end of the season.
His team made a swift return to the Premier League and enjoyed a 12-game unbeaten run during the 2009/10 season, a club record in the top division.
McLeish described the cup win over Arsenal at Wembley in February of this year as his "greatest achievement". But he said relegation was "probably the worst moment of my career" after a 2-1 defeat at Tottenham on 21 May condemned Birmingham to the drop.
If Villa do take on McLeish, it would finally bring an end to their search for a new manager.
Mark Hughes was strongly fancied for the job when he left Fulham, but the move has yet to materialise, while Rafael Benitez, David Moyes, Owen Coyle and Carlo Ancelotti have also been touted as possible appointments. Steve McClaren, another supposed contender, looks like he may take the reins at Nottingham Forest after Billy Davies was sacked yesterday.
Roberto Martinez, whose Wigan side survived on the final day of the season, turned down the chance to hold talks with Villa in favour of signing a new deal at the DW Stadium.
All eyes will be on McLeish today, as fans of Villa and Birmingham seek an explanation for his motives to walk out of St Andrew's so abruptly.
McLeish's departure as Scotland manager in November 2007, when he had two years left on his contract, was also controversial. After repeatedly denying he had been approached about a top job in England, McLeish first accompanied the then SFA chief executive Gordon Smith and president George Peat to South Africa for the qualifying draw for World Cup 2010.
However, within hours of the Scotland party returning to Britain, McLeish - who had narrowly missed out on leading his country to the finals of Euro 2008 from a group containing 2006 World Cup finalists Italy and France - quit the job and headed for Birmingham, where he was named as their new manager a day later.
Explaining his decision at the time, McLeish said: "The Scotland campaign was over and I really thought long and hard about staying on. I knew of some interest in the Premier League and had a desire to get back to day-to-day football.
"I've come to a club with a great tradition in football and a fervent and fantastic fan base. I look forward to the challenge."
It is a challenge he met head-on, first suffering relegation in 2007-08, then guiding the club to promotion at the first attempt, but succumbing to the drop again this year.
If his next challenge does happen to be Aston Villa, it could be the greatest he has faced.