The result left Milan in third place with 52 points, two behind Juve and 11 back from league leaders and bitter rivals Inter. Matteo Pessina, with a penalty on the stroke of half-time and Dany Mota strike gave mid-table Monza a 2-0 lead at the interval.
Monza appeared to be cruising to victory when Milan were reduced to 10 men in the 54th minute after Luka Jovic was red-carded for hitting an opponent. Milan coach Stefano Pioli was forced to summon heavyweights Rafael Leao, Christian Pulisic and Olivier Giroud from the bench and the changes reaped an immediate dividend.
Giroud was set-up by Pulisic to score his 12th goal of the season just after the hour with Pulisic himself making it 2-2 in the 88th minute. However, young French star Warren Bondo gave Monza the advantage again before Lorenzo Colombo added a fourth goal from in injury time.