All eyes were on the mercurial Italy striker following his £19million arrival from Manchester City and he duly delivered a clinical brace, his first goal arriving in the 24th minute.
Gianpiero Pinzi pulled Udinese level in the second half and as Milan drifted towards a draw, Balotelli's head appeared to drop. But he snatched up the ball with glee when Stephan El Shaarawy won a suspicious penalty from Thomas Heurtaux in the final minute of injury time.
Despite Udinese's angry protestations - Heurtaux appeared to make a clean tackle - Balotelli kept his head from the spot to fire his new employers into fourth place in Serie A.
Earlier, Juventus went three points clear at the top of the standings thanks to a 2-1 victory at Chievo.
An early strike by Alessandro Matri was followed by a second just before the break from Stephan Lichtsteiner, before Cyril Thereau reduced the arrears in the second half. New signing Nicolas Anelka was an unused substitute for Juventus.
Siena climbed off the bottom of the table with a shock 3-1 home victory over Inter Milan, who have now failed to win on the road since November.
Debutant Innocent Emeghara, recently signed from Lorient, stunned the Nerazzurri with his first Siena goal, only for Antonio Cassano to equalise inside 60 seconds.
An Alessio Sestu goal ensured Giuseppe Iachini's hosts led at the break.
The second half might have been a different story for Inter had Cristian Chivu not seen red for a professional foul, with Alessandro Rosina converting the resulting penalty.
A last-gasp goal from debutant Marco Rigoni secured a 3-2 victory over Lazio that lifted Genoa out of the relegation zone.
It was a well-deserved first win for new Genoa coach Davide Ballardini, whose side built a two-goal lead inside 21 minutes through Marco Borriello and Andrea Bertolacci.
Sergio Floccari and Stefano Mauri dragged Lazio back into the game, only for deadline-day signing Rigoni to fire in a bullet header for Genoa at the death.
Fiorentina got back to winning ways with a comfortable 2-0 victory over Parma.
Vincenzo Montella's men were chasing a first win in five games to restore their bid for a Champions League place and Luca Toni put them on track with the opener.
The visitors, who are sliding down the league table, were condemned to a second straight defeat when Stevan Jovetic scored five minutes into the second half.
Pescara dropped into the relegation zone after losing 3-2 at home to Bologna in an eventful contest featuring three penalties in the space of 16 first-half minutes.
The home side twice took the lead from the spot through Vladimir Weiss and Gaetano D'Agostino, with Alessandro Diamanti initially equalising with a penalty for the visitors.
Diamanti then set up a second equaliser for Alberto Gilardino before Panagiotis Kone scored the winner for Bologna midway through the second half.
Palermo's rash of January signings were unable to change their fortunes at the foot of the table as they lost 2-1 at home to fellow strugglers Atalanta.
Palermo signed a total of 10 players in January, with Diego Fabbrini and Alejandro Faurlin the last to arrive on deadline day.
They both made their debuts but ended in a losing cause as second-half goals from Carlos Carmona and German Denis put Atalanta in control before Nelson's late consolation.