Nielsen started karting in 2007 and competed through 2009. In 2010 she debuted in the Danish Formula Ford Championship, finishing ninth, and made two one-off appearances in ADAC Formel Masters before completing a full season there in 2011. She switched to GT racing for 2012 with the Porsche Carrera Cup Germany in the B-class, finishing fifth in class, then moved to the ADAC GT Masters in 2013.
Nielsen entered the United SportsCar Championship in 2014 with NGT Motorsport in a Porsche 911 GT America, adding a Petit Le Mans appearance for TRG-AMR. Her 2015 season with TRG-AMR was the most competitive of her formative years: sharing a GTD Aston Martin Vantage GT3 with Kuno Wittmer and James Davison, she took four second-place finishes and one third, landing second in the GTD standings — just two points behind champions Townsend Bell and Bill Sweedler.
The 2016 season was the defining chapter of Nielsen's career. Scuderia Corsa paired her with Alessandro Balzan in a Ferrari 488 GT3 for the IMSA WeatherTech SportsCar Championship GTD class. The pair opened the year by winning the 12 Hours of Sebring, with Jeff Segal as third driver. They added a victory at the 6 Hours of Watkins Glen and accumulated four further podiums, finishing the season first with 299 points and also winning the North American Endurance Cup GTD title. Nielsen became the first woman to win a major full-season professional sports car championship in North America.
Nielsen and Balzan returned to Scuderia Corsa in the same Ferrari 488 GT3 configuration for 2017 and successfully defended the GTD championship, accumulating 340 points. Their campaign included a race victory at Laguna Seca and a string of high-scoring results throughout the season.
In 2018 Nielsen drove a Porsche 911 GT3 R for Wright Motorsports, winning at Road America and finishing second at VIR to end the year seventh overall. The 2019 season brought a move to Heinricher Racing with Meyer Shank Racing in an Acura NSX GT3. For 2020 she ran a partial campaign for GEAR Racing powered by GRT Grasser in a Lamborghini Huracán GT3 Evo. She rejoined Team Hardpoint EBM for 2021 with a Porsche 911 GT3 R. Her FIA driver categorisation was Silver through 2021, reclassified to Bronze from 2022.
Nielsen made three starts at the 24 Hours of Le Mans. In 2016 she drove for Formula Racing in a Ferrari 458 GTE in the GTE Am class, finishing sixth in class. In 2017 she raced with Scuderia Corsa in a Ferrari 488 GTE, finishing 14th in class. Her 2018 Le Mans drive for Ebimotors in a Porsche 911 RSR produced sixth in class.
At the 2021 24 Hours of Nürburgring, Nielsen was part of the WS Racing all-female crew alongside Pippa Mann, Célia Martin, and Carrie Schreiner in an Audi R8 LMS GT4 Evo; they won the SP8 class overall. In 2022 Nielsen joined the Iron Dames programme in the FIA World Endurance Championship, driving a Ferrari 488 GTE Evo in the LMGTE Am class. She also appeared in the Intercontinental GT Challenge and the GT World Challenge America that year.
Nielsen's 2016 championship, and its 2017 defence, represent the most significant benchmark for female drivers in North American professional sports car racing. Racing throughout her career under number 88, she combined consistent points-scoring with a record of landmark victories at Sebring, Watkins Glen, and Road America across her IMSA campaigns.
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