Bayern were already crowned champions and had one eye on the Champions League final against second-placed Borussia Dortmund going into match day 34.
But despite fielding a full-strength side, the Bavarian giants went 3-1 down after just 10 minutes in the first German league fixture to see such an early goal rush in 15 years.
Javi Martinez made it 2-1 after sudden goals from Martin Stranzl and Mike Hanke, only for Havard Nordveit to restore a two-goal cushion in the 10th minute.
Franck Ribery levelled a thrilling contest with a goal either side of half-time before Arjen Robben's volley secured a winning finish to a glorious return to the top of the domestic mountain for Jupp Heynckes' side, whilst extinguishing Monchengladbach's feint hopes of snatching a Europa League place.
"I think our kind was still at the title celebrations at the start," said 1974 World Cup winner Heynckes, who is still the Bundesliga's third highest scorer. "But we stayed with it and gradually took control of the game.
"I had the feeling my players really wanted to win. I had told them before the game that this was my last Bundesliga game and it was here that I started in 1964," added Heynckes. "I saw they were really happy afterwards that they gave me such a present."
Schalke, meanwhile, went to Freiburg knowing a draw would be enough in their de facto play-off for fourth spot but took all three points in a 2-1 success.
Julian Draxler's 10th of the season put the visitors ahead but an improved second half performance from the hosts led to Jonathan Schmid equalising with 54 gone.
The hope of finding a crucial winner lasted just four minutes, however, as an unfortunate own goal went in off captain Julian Schuster and ensured the fairytale campaign for Freiburg would result in a Europa League place, but nothing more.
Bayer Leverkusen's 1-0 win at Hamburg ensured the other Europa berth went to Eintracht Frankfurt, who drew 2-2 with Wolfsburg. Both results were secured in stoppage time with Ricardo Rodriguez's own goal levelling for Frankfurt and Stefan Kiessling's 25th goal of the season securing the win for Bayer and the top scorer trophy for himself.
Robert Lewandowski looked set to share the accolade with Kiessling after putting Borussia Dortmund ahead early on against relegation-threatened Hoffenheim.
But a pair of late penalties, the latter resulting in a red card for Roman Weidenfeller and Kevin Grosskreutz taking the gloves for the closing stages, were converted by Sejad Salihovic to earn Hoffenheim a final chance to survive when they meet Kaiserslautern in a two-legged promotion/relegation play-off.
Greuther Furth were already down before the final fixtures, but their 3-1 defeat at Augsberg meant the home side secured survival at the death with Fortuna Dusseldorf's 3-0 humbling at mid-table Hannover condemning them back to the second tier after one season.
In dead rubber results, Nurnberg came from a goal down to beat Werder Bremen 3-2 despite Chelsea loanee Kevin de Bruyne's brace, and Stuttgart shared the spoils in a 2-2 draw at home to Mainz.