Controversial umpiring coupled with shoddy batting display led to India's exit from the tri-series as Australia beat them by 18 runs to storm into the finals of the DLF Cup.
Mongia, 63 not out, played the lone ranger amid another top order collapse as chasing a modest total of 214, India were bundled out for 195 in 43.5 overs.
Poor decisions -- Dhoni dismissed off a no ball and Harbhajan caught behind when the snickometer clearly showed he was not out-- at crucial stages of the match added to India's misery as a helpless Mongia watched the wickets tumble at the opposite end.
Earlier, India got off to a disastrous start after an umpiring faux pas brought about the dismissal of a distracted Sachin Tendulkar.
Tendulkar was given out caught behind off Glenn McGrath after he had made only four but television replays showed clearly that the ball had deflected from the batsman's shoulder with the bat or the gloves nowhere near it.
The Kinrara Oval witnessed an unusual drama when the master blaster was called back by the umpire immediately after being given out.
However, Tendulkar could not cash in on the slice of good luck as he was out caught by Mike Hussey at point off speedster Brett Lee in the very next over on the same score.
Thereafter, Lee dismissed Sehwag in the following over.
This brought Dravid and Kaif together who put up a brief partnership of 27 runs before Kaif was dismissed caught and bowled by Clark for 21 runs.
The bowler struck again in the same over to remove the Indian skipper and crash India's hopes of reaching the finals.
Dravid's fall brought the two left-handers Raina and Mongia together for the first time in the series.
The two put up a promising 46-run partnership to propel a brief revival before Raina surrendered to Hogg for 26 runs.
Earlier, a disciplined bowling performance by India saw the Aussies being skittled out for 213 in 48 overs.
A 36-run first wicket partnership between Hayden and Katich gave Australia a decent start before seamers struck vital blows to put India in control.
Agarkar drew the first blood when Raina took a blinder at point region off him to send Katich back for a meagre nine runs.
Following Katich's exit, Agarkar took a brilliant running catch off Munaf to send back Aussie captain Ricky Ponting for four runs.
Thereafter, Dhoni effected a crucial run-out to dismiss Hayden who was looking good after smashing a half-century.
After Hayden's exit, Mongia struck twice in two balls in his very first over getting Martyn run-out and trapping Symmonds lbw.
Following the the two quick dismissals, Haddin joined Hussey at the crease.
The feet-tied Aussies were fast running out of batsmen as Harbhajan almost effected the third run-out of the innings.
Haddin was woefully short when the throw came into the off-spinner who whipped off the bails at the non-striker's end.
Haddin was initially given out but returned back as replays showed that Harbhajan had broken the stumps without the ball in his hand.
Earlier, the Australian skipper won the toss and elected to bat against India in the do-or-die encounter.
India made two changes to the side. Dinesh Mongia and Mohammad Kaif were included at the expense of Yuvraj Singh and S Sreesanth.
Australia played a full strength side and included both Brett Lee and Glenn McGrath.
The Indians had scraped through for a 16-run victory against West Indies in their last league match to keep their hopes alive.
Australia: SM Katich, ML Hayden, RT Ponting, DR Martyn, A Symonds, MEK Hussey, BJ Haddin, B Lee, GB Hogg, SR Clark, GD McGrath
India: R Dravid, SR Tendulkar, V Sehwag, MS Dhoni, SK Raina, D Mongia, M Kaif, AB Agarkar, Harbhajan Singh, MM Patel, RP Singh