When he was 7 years old, he emigrated to Greenpoint, Brooklyn in 1996 along with his parents and brothers, settling into the large Polish community there.

As an overweight youngster who spoke English with a Polish accent, he was frequently the target of bullies at school and on the streets, which gave him further motivation to enter the boxing gym.

[5] Kownacki made his professional debut on 30 October 2009, at the PAL Gym in New York, United States, where he knocked out Carossee Auponte in the first round.

His next fight took place on 29 May, where he beat Ytalo Perea by unanimous decision over 8 rounds, the first time he had gone the distance in his professional career.

Kownacki fought twice in 2016, with a unanimous decision over Danny Kelly and a knockout win against Jesse Barboza.

[6] Kownacki's first fight of 2018 was against Iago Kiladze, at the Barclays Center, New York, on the undercard of Errol Spence Jr. vs Lamont Peterson.

[8] In his second bout of 2019, on 3 August at the Barclays Center, New York, Kownacki defeated Chris Arreola by unanimous decision (118–110, 117–111, 117–111), winning the IBF Inter-Continental heavyweight title.

[12] After over a year out of the ring, Kownacki returned on 9 October 2021 on the undercard of Tyson Fury vs. Deontay Wilder III, against Robert Helenius in a rematch of their previous 2020 fight.

The former had inflicted his opponent with a swollen left eye, before suffering repeated low blows, which caused Kownacki to be disqualified in the sixth round.