Macedonian striker Goran Pandev slotted in a scuffed shot from Samuel Eto'o after six minutes to grab his third goal since rejoining Inter last month.
Defender Walter Samuel then powered in a header from a corner and Argentina's Diego Milito netted his 14th goal of the league campaign after a fine move just after the break.
"We can definitely improve a lot more but I'm happy for the three points. We are only thinking about ourselves, other results don't bother us," Pandev, whose side can extend their eight-point lead at Parma on Wednesday, told Sky TV.
AS Roma stole second place from AC Milan thanks to a fortunate 1-0 win at mid-table Fiorentina, who were denied by a tight offside call when Juan Vargas netted and spurned several other chances.
The visitors snatched victory when Mirko Vucinic fired in off the bar following a corner eight minutes from time.
Roma striker Francesco Totti, considering coming out of international retirement for Italy at June's World Cup, went off at halftime after his fitness problems reoccurred.
Milan failed to win for the fourth consecutive game in a 0-0 draw at struggling Bologna where Mancini, who signed on loan from Inter on Monday, made his debut in place of David Beckham.
NAPOLI LOSE
The Rossoneri hit the bar twice but their creative problems remain as they continue to miss injured forward Alexandre Pato.
Beckham was dropped having played all five leagues games since arriving for a second loan spell from Los Angeles Galaxy.
A fight between fans before kick-off and a scuffle in the players' tunnel marred Antonio Di Natale's hat-trick in lowly Udinese's 3-1 win over fourth-placed Napoli, who had goalscorer Christian Maggio sent off on half-time for diving.
The defeat was Napoli's first in the league for 16 games.
Italy forward Di Natale, Serie A's top scorer with 16 goals, netted early when he knocked in the rebound after his penalty was saved and he made it 2-1 in stoppage time with an acrobatic finish before adding a third.
Seven people, including a policeman, received minor injuries in trouble outside the ground in Udine, media reported.
Sampdoria's 2-1 win at bottom side Siena meant the Genoa club climbed to sixth spot, above free-falling Juventus who drew 1-1 at Livorno on Saturday.
Genoa beat fellow mid-table side Chievo 1-0 and Catania boosted their hopes of survival and left Lazio third from bottom thanks to Maxi Lopez's second-half goal in a 1-0 win in Rome.
Also at the bottom, Atalanta beat Bari 1-0 at home.
