Pioli, who celebrated his 54th birthday on Sunday, succeeded Marco Giampaolo at the San Siro earlier this month and has been charged with the task of improving the fortunes of a club which had lost four of their opening seven Serie A games.
It looked as though Pioli's reign would get off to the perfect start thanks to goals from Hakan Calhanoglu and Krzysztof Piatek either side of Khouma Babacar's leveller, only for Marco Calderoni to produce a stunning equaliser in the 92nd minute.
Milan made a bright start as Rafael Leao forced a save from Lecce goalkeeper Gabriel in the first minute after being set up by the influential Calhanoglu, who then drilled a left-footed shot from the edge of the area just over the crossbar.
It was no surprise when the home side took the lead in the 20th minute as Calhanoglu took down a long pass from Lucas Biglia on the right-hand side of the area and lashed a fierce volley into the far corner.
Suso's shot from a similar position was then tipped around the post by Gabriel as Milan pushed for a second goal, but Lecce were gifted an equaliser shortly after the hour mark.
With the aid of VAR, Andrea Conti was adjudged to have handled the ball as he failed to make a clearing header and although Babacar's weak penalty was easily saved by Gianluigi Donnarumma, the Senegal striker was able to poke home the loose ball from close range.
Milan responded well and Calhanoglu's volley flew agonisingly wide of the post before the same player surged into the area and pulled the ball back perfectly for Piatek to slot home in the 82nd minute.
That looked to have settled the game in Milan's favour but six minutes of stoppage time gave Lecce renewed hope and they stunned the San Siro with an superb equaliser.
Calderoni had been a peripheral figure throughout the contest but took aim from 25 yards and rifled a left-footed shot into the far corner beyond Donnarumma's despairing dive.