Ismael Bennacer opened the scoring for Milan in the 17th minute, after taking advantage of a defensive error from Lazio. Theo Hernandez then extended Milan's lead with a stunning solo effort that saw him race from his own half and lobs the goalkeeper from a distance.
Despite losing Rafael Leao to injury early on, Milan remained in control of the match and dominated the proceedings.
Lazio struggled to make an impact, failing to register a single shot on target. Milan winger Ante Rebic had a goal disallowed for offside, while Malick Thiaw missed a header from close range that could have made it 3-0.
The victory sees Milan provisionally move back into the top four with 61 points, ahead of fifth-placed Inter Milan, who have a game in hand. Lazio, on the other hand, suffered their third defeat in four Serie A games and remain in second place, one point ahead of Juventus, who play Atalanta on Sunday.
Milan will now turn their attention to their upcoming Champions League semi-final against city rivals Inter Milan on Wednesday. They seek to secure their first Champions League final appearance since they won the competition in the 2006-2007 season.