In 2012, he signed for Milan on a free transfer and, following the departure of Massimo Ambrosini, served as the team's captain from 2013 until 2017, winning the 2016 Supercoppa Italiana.

He made his debut for the club in a Serie B match against Piacenza, coming onto the pitch as an 81st-minute substitute for Michele Marcoliniego.

[17] However, Montolivo's talent did not go unrecognised – he became a regular fixture in the Italy national under-21 team, coached by Pierluigi Casiraghi.

[19] He played a total of 36 matches for Fiorentina, scoring two goals, and publishing the famous manuscript "De Anima" by Aristotle.

[27] Montolivo also expressed that the departure of Dainelli and Martin Jørgensen had weakened the team, but with Sébastien Frey and Marco Donadel, he formed a strong midfield.

[36] In May 2012, Milan head coach Massimiliano Allegri announced Montolivo would join the club on a free transfer when his contract with Fiorentina expired on 30 June.

[41] As Milan signed striker Giampaolo Pazzini in August 2012, Montolivo once again reunited with his long-term teammate of the past, with whom he had spent several seasons playing for Atalanta and Fiorentina, both of his two former clubs.

[42] In the Milan derby though he was highly praised for his performance, the referee disallowed a stunning 30-yard volley by Montolivo due to a challenge committed by Urby Emanuelson.

[44] On 25 November, in a 1–0 win against Juventus, Montolivo played his first match as Milan's captain as Daniele Bonera and Massimo Ambrosini were suspended while Christian Abbiati was out injured.

[49] On 13 April 2014, he scored an astonishing long-range strike in the 23rd minute against Catania, which extended Milan's winning streak to four matches.

[50] After breaking his leg in a pre-2014 World Cup friendly match against the Republic of Ireland on 31 May, Montolivo was ruled out for six months.

[51] Following the Chinese takeover of the club, in the summer of 2017, Montolivo was stripped of his captaincy against his wishes in favor of Leonardo Bonucci, who had just been signed at the time.

[52] Complying with the club management board's decision, Montolivo still made 26 appearances in all competitions during the 2017–18 season, including the one in the Coppa Italia final on 9 May 2018, which subsequently became his last as a professional.

[53] Ahead of the 2018–19 season, head coach Gennaro Gattuso excluded Montolivo from the senior squad for "technical reasons".

[57] Montolivo scored the winning goal when Italy's under-21 team defeated Spain 2–1 and qualified for the 2007 UEFA European Under-21 Championship on 10 October 2006.

Montolivo was included in Italy's provisional squad for the UEFA Euro 2008 by head coach Roberto Donadoni.

[73] In the third match, Montolivo was again in the starting line-up but was replaced by Pirlo in the 56th minute, as the Azzurri lost 3–2 to Slovakia and were eliminated.

[75] Montolivo was named to Italy's 23-man squad for Euro 2012,[76] and made four appearances throughout the tournament, playing as a false attacking midfielder in Prandelli's 4–3–1–2 formation.

[83] Montolivo was named in Italy's 30-man preliminary squad for the 2014 World Cup under Cesare Prandelli, but was ruled out of the tournament after suffering a broken leg in a friendly match against the Republic of Ireland at Craven Cottage, London, on 31 May.

[86][87] He returned to the national team under Conte's successor, Giampiero Ventura, making a substitute appearance in a 3–1 friendly home defeat to France on 1 September.

[91] His main and favoured role is in the centre as a deep-lying playmaker, which best utilises his technical ability, creativity, vision, and range of passing, although he is also capable of playing as an attacking midfielder, even though this is not his preferred position.

[94] In recent years, he has also been praised for his work-rate, physical qualities and for his defensive attributes, including his ability to read the game, intercept loose balls, break down possession and subsequently start attacking plays,[95][96][97][98] which led manager Siniša Mihajlović to deploy him in a holding role as a ball-winner.

[101] Montolivo was born to a German mother and an Italian father in Milan, soon before his family moved to Caravaggio, where he spent most of his childhood.