Samu Castillejo put the Rossoneri ahead in the 26th minute but Lecce equalised through Marco Mancosu’s penalty after Matteo Gabbia fouled fellow substitute Khouma Babacar.
However, Giacomo Bonaventura and Ante Rebic, who was sent off in the Coppa Italia semi-final exit at the hands of Juventus, struck in the next three minutes to put Milan out of reach.
Substitute Rafael Leao put the result beyond doubt in the 72nd minute as the Rossoneri returned to seventh after Hellas Verona and Parma moved above them at the weekend.
Milan found Lecce goalkeeper Gabriel in good form during the opening 25 minutes, the Brazilian denying Bonaventura, Castillejo and Theo Hernandez.
There was little Gabriel could do when Castillejo instinctively steered Hakan Calhanoglu’s drilled, low cross into the bottom-left corner for his first Serie A goal since April 2019, though.
Lecce were allowed back into the game and Biagio Meccariello had a goal ruled out for a narrow offside.
Babacar replaced Gianluca Lapadula for the second half and he won a penalty off Gabbia – a first-half replacement for the injured Simon Kjaer – that Mancosu coolly converted in the 54th minute.
Milan regained their advantage just 63 seconds later when Gabriel parried Calhanoglu’s shot straight to Bonaventura, who slotted home on the follow-up.
The Rossoneri increased their lead with a swift break from a Lecce corner in the 57th minute, Rebic latching onto Calhanoglu’s pass and beating Gabriel.
Leao was sent on for Rebic in the 68th minute and headed home a cross from Andrea Conti to cap a resounding win for Milan.
What does it mean? Milan find goals from multiple men
Stefano Pioli’s side were not at their best but four goals from as many scorers will be pleasing for the coach, more so in the absence of Zlatan Ibrahimovic.
It was the first time Milan have scored four in a single league game this season and they will hope to build on that with a run of tricky fixtures in the coming weeks.
Calhanoglu the creative hub
Despite playing primarily from the left, Calhanoglu was central to some of Milan’s brightest moments. He assisted two goals and played an important role in the other, creating a total of five chances for his teammates.
Gabbia struggles at the back
Milan looked shakier in defence following the introduction of Gabbia and his foul on Babacar gave Lecce a route back into the game.
Milan host Roma on Monday in a game that could prove pivotal in their quest for European qualification, while Lecce continue their battle against relegation with a trip to Juventus on Saturday.