In 1986–87, he joined fellow league side A.C. Milan, being first-choice for three of his four years, and establishing himself as one of Italy's best goalkeepers, winning the Serie A title and the Supercoppa Italiana during his first season.

In his final season with the club, he was often alternated and faced competition for a starting spot from back-up Andrea Pazzagli, whom manager Arrigo Sacchi often fielded during league games, while Galli played in European matches.

[1][2] Galli took part in Italy's victorious 1982 FIFA World Cup expedition, without playing a single match however, as he served as backup to Dino Zoff, alongside Bordon.

[2] As a starter, Galli played in the 1986 World Cup in Mexico under manager Enzo Bearzot,[1][4][5] as Italy were ousted in the round of 16 by eventual semi-finalists France, led by Michel Platini.

[6] During the tournament, in Italy's 1–1 draw against eventual champions Argentina in their second group match on 5 June, Galli drew criticism in the media for his goalkeeping on Maradona's equaliser.

He was known in particular for his strong positional sense, handling, and his efficient rather than spectacular playing style, as well as his calm personality and serious demeanour, both on and off the pitch; however, he was also an athletic shot-stopper, with a good spring, who was capable of producing decisive diving saves when necessary.

In spite of his reserved character, he was also highly regarded for his charismatic leadership, as well as his mentality, and his ability to communicate with his defenders and organise his back-line, which often inspired a sense of confidence in his teammates.