[2] She made her Olympics debut at the 2012 games where she was taking part in the 100 and 200 m backstroke and the 4×100 m relays in freestyle and medley.

[1] Nielsen missed the 2013 World Aquatics Championships with a knee injury, but won the bronze medal in the 100 m backstroke at the 2015 World Aquatics Championships in Kazan, Russia.

[3] At the 2016 Summer Olympics in Rio de Janeiro, she won a bronze medal as a part of the 4 × 100 m medley relay alongside Rikke Møller Pedersen, Jeanette Ottesen and Pernille Blume.

In October 2020 Nielsen announced her retirement from competitive swimming, citing continued shoulder problems and the uncertainty regarding the proponed 2020 Summer Olympics.

[4] She is the daughter of Benny Nielsen, who won silver in the 200 m butterfly at the 1988 Summer Olympics and Lone Jensen, who participated in the 1978 World Aquatics Championships.