Despite their side having little to play for, a small group of home fans did their best to rouse the Black and Greens, making plenty of noise in response to a flat start from both teams.
That excitement on the terraces was nearly rewarded when Nedim Bajrami slid wide from inside the area, while the forward also had a shot beaten away by Michele Di Gregorio, who then saved Domenico Berardi’s rebound.
Soon afterwards, a tight offside meant a penalty awarded in Sassuolo’s favour was overturned, before Patrick Ciurria put a rare Monza shot over the bar.
The bulk of the chances were at the other end though, and after Bajrami had seen a shot blocked narrowly wide, VAR awarded the hosts a harsh spot-kick for handball against Armando Izzo, and it was duly dispatched by Berardi deep into added time.
Given the White and Reds had only picked up one win from the 20 previous Serie A games in which they had conceded first, they were not fancied to find a way back into the match.
However, Raffaele Palladino’s side found an equaliser out of nowhere on the hour mark when they netted with their first shot on target of the match as Ciurria converted into the roof of the net from close range.
The visitors were given a further boost when Ruan Tressoldi picked up his second yellow card as Sassuolo were reduced to 10 men, and Monza nearly made that man-advantage count straight away when only a superb save stopped Ciurria from netting his second.
Time appeared to be running out for a winner after Samuele Vignato fired over from distance and Samuele Birindelli missed a golden chance from yards, but Pessina slotted home in the 93rd minute to seal the victory, before sprinting the full length of the pitch in celebration.
The victory leaves Monza in eighth place in the table, six points behind seventh-placed Atalanta with two games remaining.
Flashscore Man of the Match: Matteo Pessina (Monza)