Tonali made his professional debut for Brescia Calcio on 26 August 2017, aged 17, coming on as a substitute in the Serie Baway match lost 2–1 against Avellino. On 28 April2018 he scored his first professional career goal, in a 4–2 defeat against Salernitana in Serie B. He obtained 19 league appearances in total during his first season as a professional player, scoring two goals and providing two assists during the 2017–18 Serie B season. The following season, Tonali won the Serie B title with Brescia, and achieved promotion to Serie A, featuring as a starter for the club throughout the entire 2018–19 season. He made his Serie A debut on 25 August 2019, aged 19, in a 1–0 away win against Cagliari. On 29 September, he assisted Mario Balotelli's goal from a corner kick in a 2–1 away loss to Napoli; he also scored a goal during the match with a powerful shot from outside the area, which was disallowed by VAR, however, for a foul by Brescia's Dimitri Bisoli on Napoli's Nikola Maksimović. He scored his first Serie A goal on 26 October, netting a long-range free kick into the top corner from the left flank, which was the opening goal in an eventual 3–1 away defeat to Genoa.
Regarded as a promising young Italian player in his position in the media, Tonali is a regista, and is 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. A strong, energetic, elegant, and creative, midfielder, with excellent vision and an ability to read the game, Tonali usually plays in a central or defensive midfield role as deep-lying playmaker in a 4–3–3 formation, and shows maturity and character beyond his young age. Dictating play for Brescia with his passing in midfield, he set up two goals and even scored two himself in his 19 appearances in Serie B during the 2017–18 season. Although naturally right-footed, he is known for his accurate long passing ability with either foot. 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. In July 2019, he was named by UEFA as one of the 50 promising young players to watch for the 2019–20 season.
Career statistics
Club
International
Honours
Club
Brescia
Serie B: 2018–19
International
Italy
UEFA European Under-19 Championship runner-up: 2018
Individual
UEFA European Under-19 Championship Team of the Tournament: 2018