The club secured the Italian title a week ago when it drew 0-0 away to Roma, and since then played the Italian Cup semifinal away to Palermo.
Ahead of the match, the team went on an open top bus ride through city streets thronged with supporters, before leading a procession to the San Siro.
Once at the stadium, players, their wives and children celebrated in front of the fans on the pitch wearing commemorative T-shirts.
When the match finally started, a brace from Robinho and a strike by Gennaro Gattuso gave Milan a 3-1 lead at the break before Clarence Seedorf added a fourth in the closing stages.
At full time the celebrations continued with the trophy presentation on the field in front of a capacity crown that included club president and Italian Prime Minister Silvio Berlusconi.
"We really wanted the guys to finish well at home, in front of the fans," Milan coach Massimo Allegri said.
"They all did their best and deservedly finished with a win." Also Saturday, Lazio scored three goals in the second half to keep up its pursuit of a Champions League place with a 4-2 win over Genoa.
At the San Siro, Cagliari's Radj Nainggolan had the first effort of the match, but shot wide from long range.
Milan struggled to create chances early on, but Robinho finally opened the scoring in the 22nd minute.
The Brazilian raced in from the left wing, slalomed past three defenders and curled a shot past goalkeeper Michael Agazzi from the edge of the box.
Two minutes later, Gattuso doubled Milan's lead. Robinho played a through ball toward Alexandre Pato, but Agazzi appeared to snuff out the danger as he rushed off his line.
However, he fumbled the ball and as he tried to clear it went straight to Gattuso who headed into the empty net.
Robinho doubled his tally in the 35th. Pato slipped ball to him, the Brazilian took one touch and placed a shot under Agazzi.
Cagliari pulled a goal back three minutes later when Andre Cossu cut in past Mario Yepes and shot under Milan keeper Christian Abbiati.
With the clock ticking down, Seedorf ran onto another through ball from Pato to poke home his third goal of the season and wrap the match up for Milan.
"Tonight we saw the level we need to play at against a team like Milan," Cagliari coach and former Milan player Roberto Donadoni said. "You can't switch off for a moment or you're suddenly 2-0 down like we were. We did so again and so were 3-0 down." Lazio was quickly into its stride and ahead after six minutes when Giuseppe Biava rose highest to head in a corner.
The lead didn't last long though and Rodrigo Palacio equalized when he was first to the rebound after Fernando Muslera had saved Mauro Bosseli's shot.
Ten minutes into the second half Lazio was two goals ahead and the match effectively over. Lazio's second came when Cristian Ledesma played a ball over the defense to Tommaso Rocchi, who took one touch and slipped the ball under Eduardo.
Hernanes then headed in Ledesma's cross in the 55th for Lazio's third and the Brazilian struck again in the 66th, when he swept home Mauro Zarate's cutback from the byline.
Antonio Floro Flores scored a consolation second for Genoa in the 89th minute.
"We did what we had to do," Lazio coach Edy Reja said.
"Today's victory gives us a lot more hope. We now have 63 points and I hope we that we can finish with at least a place in Europe."
