Justyna Święty-Ersetic competed in the 4 × 400 metres relay at the 2012 London and 2016 Rio Summer Olympics.

At the 2018 European Championships in Berlin, she won an individual gold and later also anchored the Polish squad to the win despite the fact that both finals were separated by less than two hours.

At the 2020 Tokyo Olympics she represented Poland in the 4 x 400 m mixed relay, winning gold alongside Karol Zalewski, Natalia Kaczmarek and Kajetan Duszyński.

A few days later, Święty-Ersetic anchored the women's 4 × 400 m relay (Kaczmarek, Iga Baumgart-Witan and Małgorzata Hołub-Kowalik), sealing a silver for the group.

[4][5] In March 2022, Święty-Ersetic won for the second time World Athletics Indoor Tour in her specialist distance after her 2020 victory, breaking the meeting record in Madrid.