Born in Bergamo to physical education teacher and a Sicilian mother, Gagliardini began his career at hometown club Atalanta playing for their youth teams and later for the reserves.

[2] On 4 December 2013, Gagliardini made his professional debut, starting in a 2–0 home win over Sassuolo, for the campaign's Coppa Italia.

[23][24] He was presented two days later, where he was assigned the squad number 5 (previously worn by Felipe Melo, who left Inter in the same transfer window).

[25][26] Gagliardini made his Inter debut on 14 January against Chievo at San Siro, playing the full-90 minutes in a 3–1 win to help the Nerazzurri claim their fifth consecutive league victory.

[31] On 12 August 2015, he made his debut with the under-21 side under manager Luigi Di Biagio, in a friendly match against Hungary, which ended in a 0–0 draw.

[35][36] On 28 March 2017, Gagliardini made his first senior appearance for the Italy national football team, along with four other players; he came off the bench to replace the injured Daniele De Rossi in the 37th minute of a 2–1 friendly away win against the Netherlands.

[40][41] At 1.90 m (6 ft 3 in) tall, Gagliardini is a complete midfielder, known for his physical attributes, work-rate, vision, ability to read the game, and technique.