Marchwiński is a youth product of Lech Poznań, and was registered to their first team squad on 12 December 2018.[2] On 16 December 2018, he made his professional debut with Lech Poznań in a 6-0 Ekstraklasa away win over Zagłębie Sosnowiec and scored his team's final goal, becoming the youngest Ekstraklasa scorer in club's history, at the age of 16 years and 340 days.[3]
On 25 July 2024, Marchwiński travelled to Italy to undergo a medical with Serie A club Lecce.[4] The following day, he signed a four-year contract, with an option for a further year. The transfer fee was not disclosed, and reported to be €3 million, with an additional €1 million in bonuses.[5]
International career
Marchwiński has been a part of Poland youth system since as early as 2014, when he received his first call-up to Poland U13s.[6] He scored twice in his only appearance for the under-14 squad in a 5–0 win against Slovakia on 17 November 2015.[7]