Mikołajczyk took up canoeing in 1996 in her native Bydgoszcz. She won the silver medal in K-2 500 m event at the 2008 Summer Olympics in Beijing with Aneta Pastuszka-Konieczna. She won the bronze medal in K-2 500 m event at the 2012 Summer Olympics in London with Karolina Naja. She also won a gold in the K-2 1000 m event at the 2009 ICF Canoe Sprint World Championships in Dartmouth.