Having started out at local side GKP Gorzów Wielkopolski, Kownacki joined Lech Poznań in 2005, coming through the youth ranks and making it to the first team in December 2013. He scored his first Ekstraklasa goal in February 2014 in a 5–1 loss at Pogoń Szczecin, becoming one of only five players aged under 17 to have registered in Poland's top flight. He won his first league title the following season. In 2017 he commented: "At 16, people were calling me 'the new Lewandowski' which was tough to handle, but I changed my attitude, grew up and am now much better at not taking to heart what people say. Of course, I still have big dreams".[4]
Sampdoria
On 11 July 2017, Kownacki signed a five-year contract with Italian side Sampdoria.[5]
On 31 January 2019, Kownacki joined to German Bundesliga club Fortuna Düsseldorf on loan with an option to buy.[7] On 30 June 2019, he rejoined Fortuna on another loan with an obligation to buy,[8] which was fulfilled in January 2020.
On 2 February 2022, Kownacki returned to Lech Poznań on loan until the end of the season.[9]
Werder Bremen
On 22 May 2023, Bundesliga club Werder Bremen announced the signing of Kownacki on a free transfer from Fortuna for the 2023–24 season.[10]
Kownacki represented Poland at various youth levels ranging from U16 until U21, earning 46 appearances and scoring 31 goals.[citation needed] He received his first call-up to the senior squad for matches against Georgia and Greece in June 2015.[12] On 23 March 2018, Kownacki debuted in a friendly match against Nigeria, playing 72 minutes.[13]