Petagna started his football career in A.C. Milan's youth system, where he was a member of the under-15 squad who won the Campionato Nazionale Giovanissimi in 2010 and a member of the under-17 squad who won the Campionato Nazionale Allievi the following year. He made his professional debut on 4 December 2012, coming on as a substitute in the late stages of a UEFA Champions League group stage home game against Zenit St. Petersburg, which Milan lost 0–1. At the start of the 2013–14 season, Petagna was promoted full-time to the first team. He made his Serie A debut on 24 August 2013, coming on as a late substitute in the opening game, a 2–1 away loss against newly promoted Hellas Verona. However, following the acquisition of Alessandro Matri in the late days of the summer transfer window, Milan decided they would let Petagna go out on loan to Sampdoria in order to gain more first team experience. Sampdoria also had the option to make the deal a co-ownership agreement at the end of the season-long loan spell. In spite of that, the loan was terminated early and Petagna returned to Milan in January, having made five scoreless appearances during his stay at Sampdoria. He finished the season with three more senior appearances, while being mainly employed by the under-19 squad managed by Filippo Inzaghi. The side won the Viareggio Cup in February, defeating Anderlecht in the final, with Petagna scoring the equaliser for the rossoneri in a 3–1 win. On 16 July 2014, Petagna was loaned out to Serie B sideLatina ahead of the 2014–15 season. On 13 January 2015, he was signed by Vicenza on loan. On 30 August 2015, he was signed by Serie B newcomers Ascoli on loan.
On 25 January 2016, Petagna signed for Atalanta and was loaned to former club Ascoli until 30 June 2016.
SPAL
On 19 July 2018, Petagna signed for SPAL on loan until 30 June 2019 with an obligation to buy. He scored his first goal with S.P.A.L. in his debut match on 12 August, scoring the final 1-0 goal in the Coppa Italia third round match against Spezia He scored his first Serie A goals with the Emilian side on 17 September, scoring a double that allowed the Biancazzurri to beat his former side Atalanta 2-0. He made 37 appearances scoring 17 goals in all competitions, On 1 July 2019, Petagna was signed on a permanent deal by SPAL.
Napoli
On 30 January 2020, Petagna signed with Napoli. He will stay at SPAL on loan until the end of 2019–20 season.
