The striker struck in the 90th minute to pile more misery on Inter who appeared to be on the brink of a much needed victory
Nicolai Lopez scored an injury-time equaliser as Verona snatched a last-gasp point in a 2-2 draw with Inter at San Siro.
Luca Toni scored an early opener but Icardi volleyed a first-half equaliser and then tapped in a second to turn the game on it's head.
But the drama was far from over as Gary Medel received his marching orders for handling in the area, only for Samir Handanovic to save yet another penalty.
However, there was nothing he could do to stop Lopez from scoring a late equaliser, which makes it three games without a win for Inter.
Verona made a much stronger start and went ahead with a slick counter-attack - led by the returning Emil Hallfredsson - that saw Lazaros Christodoulopoulos nod into the path of Toni and the veteran striker expertly volley home from six yards.
The hosts rallied and equalised in almost identical fashion in the 18th minute as a deep cross was nodded into the centre by Rodrigo Palacio and Icardi was on hand to volley in from close range.
It lifted Inter and they almost completed the turnaround moments later as Zdravko Kuzmanovic's 25-yard curling shot cannoned off the post.
The end-to-end nature of the game continued with a superb reaction save from Rafael denying Nemanja Vidic's powerful header before an inswinging Christodoulopoulos cross evaded everyone but the razor-sharp Handanovic in Inter's goal.
It took less than three minutes of the restart for Inter to take the lead as Palacio found space and drilled to the far post where Icardi tapped in his seventh Serie A goal this season.
Verona had a chance to draw level from the spot after 50 minutes when Medel handled Juanito's shot, earning him a second booking and an early bath.
However, Handanovic guessed right and saved Toni's penalty to register his third spot kick save of the season.
Christodoulopoulos hit the bar and Toni clipped wide from close range as the visitors pressed for an equaliser, which finally arrived as Lopez linked up with fellow substitute Javier Saviola and slammed home.
Mazzarri trudged straight down the tunnel as the final whistle blew, with speculation sure to rage about his job security throughout the week.