Tonali made his professional debut for Brescia Calcio on 26 August 2017, aged 17, coming on as a substitute in the Serie B away match lost 2–1 against Avellino.

[14] Tonali made his competitive debut for Milan on 17 September 2020, coming on as a substitute in the 83rd minute in a 2–0 victory against Shamrock Rovers FC in the second round of Europa League qualifying.

[17] On 3 October at Gewiss Stadium against Atalanta, Tonali dispossessed the ball from Remo Freuler and ran to the opponents box to score the second goal in eventual 3–2 victory for Milan.

[19] On 24 April as the match against Lazio was heading to 1–1 draw, Tonali scored the winner in the stoppage time, securing the lead for his team in Serie A table.

Tonali is also the youngest midfielder to have scored five or more goals in Serie A that season (and the second-youngest among all players, older only than Bologna's Aaron Hickey).

[22] After 3–0 away win against Sassuolo on 22 May, Milan secured their first Scudetto in 11 years, In total, Tonali played 36 games in the campaign, scoring 5 goals and providing 3 assists.

[34] On 28 August 2024, after serving his suspension, Tonali made his first appearance since October the previous year, playing 61 minutes of a 1–1 draw against Nottingham Forest in the third round of the EFL Cup.

[43] Regarded as a promising young Italian player in his position by the media, Tonali has been described by pundits as a regista,[44] although he is a versatile midfielder, who is capable of playing in several roles.

[45] In his youth, he was often likened to Andrea Pirlo – who also started his career with Brescia – in terms of his movement, coordination, appearance, technical qualities, position, and playing style.

[53] His tenacity and defensive contribution also allow him to play as a box-to-box midfielder, while his creativity, ability to find space, make late runs, and his penchant for driving forward with the ball even saw him deployed in a more advanced role on occasion at Milan under manager Stefano Pioli, namely as a mezzala.

[54][55] In addition to Pirlo, Tonali has also cited Steven Gerrard and Luka Modrić as influences, and has also compared his own playing style to that of Gennaro Gattuso.