[11] Three days before his 16th birthday, in February 2015, he received his first call-up to the senior team from manager Filippo Inzaghi; though he did not feature in the league match against Cesena, his presence on the substitutes' bench had required a special dispensation due to his age.

[14][15] At the beginning of the 2015–16 season, Donnarumma was promoted to the senior team by manager Siniša Mihajlović, initially as the third-choice goalkeeper behind Diego López and Christian Abbiati.

Replacing López in the 72nd minute, he kept a clean sheet for the remainder of the match, but was one of two Milan players to miss his penalty shoot-out attempt in an eventual 10–9 loss.

[17] Donnarumma made his competitive debut in Serie A on 25 October against Sassuolo at the San Siro stadium; chosen to start ahead of López, he helped the team achieve a 2–1 victory after three matches without a win.

[18] At the age of 16 years and 242 days,[19] he was the third-youngest goalkeeper to start a match in the history of Italian football; 13 days older than Giuseppe Sacchi, who coincidentally made his Serie A debut with Milan on the very same date, 73 years earlier;[20][21] and two months older than Gianluca Pacchiarotti [it], who made his Serie A debut with Pescara in 1980.

[27] His first appearance in the Derby della Madonnina, Milan's crosstown rivalry with Internazionale, came on 31 January; the clash ended in a 3–0 victory after he stopped Éder from scoring.

[29] Donnarumma featured ahead of Abbiati in Milan's 1–0 defeat to Juventus in the Coppa Italia Final, as the club failed to qualify for European competition at the end of the season.

[30] In the club's opening league match of the 2016–17 season against Torino, on 21 August, Donnarumma saved the first penalty of his professional career; he stopped Andrea Belotti's shot in injury time, which enabled Milan to seal a 3–2 victory at home.

[33] On 23 December, Donnarumma helped lead Milan to victory in the Supercoppa Italiana, saving Paulo Dybala's penalty in a 4–3 shoot-out win following a 1–1 draw after extra time.

[37] Donnarumma played his first European game in a 1–0 win against Universitatea Craiova in the first leg of Milan's Europa League third-round qualification match on 27 July 2017, in which he managed to keep a clean sheet.

[43] On 28 February 2018, following a 0–0 aggregate draw after extra-time, Donnarumma saved two penalties in a 5–4 shoot-out victory over Lazio in the second leg of the Coppa Italia semi-finals in Rome, which allowed Milan to advance to the final of the competition.

[44] On 15 April 2018, Donnarumma became the youngest player to reach 100 appearances in Serie A, aged 19 years and 49 days, after a 0–0 home draw against Napoli.

[54] On 21 July 2020, he played his 200th game for the club in a 2–1 away win over Sassuolo in Serie A, during which he wore the captain's armband for the first time in his career, after Alessio Romagnoli was subbed off due to injury.

[60] On 16 June 2021, Donnarumma agreed to a five-year contract with Ligue 1 club Paris Saint-Germain (PSG), as reported by Fabrizio Romano and The Guardian.

[77] On 1 September, in the match against France, Donnarumma made his senior debut, subbing out Gianluigi Buffon at the half-time mark of a 3–1 home loss; the youngest goalkeeper to ever play for Italy aged 17 years and 189 days.

[80] Widely regarded as the heir of Buffon, following the international retirement of the latter in 2018, Donnarumma was chosen as the first-choice goalkeeper by the new head coach Roberto Mancini and played as a starter in all the matches of the 2018–19 UEFA Nations League disputed by the Azzurri.

[86] For his performances throughout the competition, including keeping three clean sheets, making nine saves, and conceding only four goals in seven appearances, he was named Player of the Tournament by UEFA,[4] the first time a goalkeeper had won the award.

Long considered one of Italy's most exciting prospects, and one of the most promising young footballers of his generation, Donnarumma is widely regarded as the successor to Gianluigi Buffon.

[96] Predicting an "extraordinary career" in his future,[97] he commended Donnarumma's maturity and ability to cope with the pressures of being a goalkeeper at such a young age.

[104] Although Donnarumma is regarded as being competent with the ball at his feet, pundits such as Mina Rzouki, Paolo Menicucci, and Sam Lopresti, as well as former goalkeeper Fernando Orsi, have cited his ball control and distribution, as well as his overall consistency, positioning, handling, punching technique, and his ability to defend crosses and command his box as areas in which he needs to improve as he gains more experience.