Kimiläinen entered professional motorsport in 2005 in the Northern European Formula Ford Championship. She scored a string of rookie awards in her debut season and finished fifth overall. In 2006 she tied on points with series champion Sami Isohella but was classified second on countback, losing by one win. She spent 2007 racing Radicals in Sweden.
Audi signed Kimiläinen in 2008, placing her in their Formula Masters series alongside future Formula One driver Kevin Magnussen. She finished tenth in the standings, with a second-place result at Assen as a highlight. The 2008 financial crisis prompted Audi to cut her funding, and she self-funded a 2009 campaign in Formula Palmer Audi, finishing fifth with four podium finishes. With backing exhausted and personal funds depleted, she stepped away from racing after 2009 and started a family, taking an indefinite hiatus from the sport.
In 2014, Kimiläinen received a call-up from PWR Racing to contest the full Scandinavian Touring Car Championship season, becoming the first woman to race in the STCC since Nettan Lindgren-Jansson in 1999. She was retained for 2015 and 2016, though a crash at the opening round at Skövde in 2016 left her with a strained neck and prevented her from completing that season. She moved to the Swedish V8 ThunderCar series in 2017.
Kimiläinen was selected as one of the 18 permanent drivers for the inaugural 2019 W Series season. She qualified strongly for the opening round at Hockenheimring but was caught up in a collision with Megan Gilkes that aggravated her previous neck injury, sidelining her for two rounds. On her return at the Norisring she finished fifth, then delivered a grand slam at Assen — pole position, fastest lap, and race victory. At the season finale at Brands Hatch she fought Alice Powell throughout the race, finishing second and leapfrogging Fabienne Wohlwend to take fifth in the championship.
The coronavirus pandemic cancelled racing in 2020. Kimiläinen returned for the 2021 W Series season and had her strongest championship campaign, winning at Spa-Francorchamps and collecting four additional podiums to finish third in the standings. In 2022 she participated in the Race of Champions, replacing Valtteri Bottas — who was prevented from competing by his Alfa Romeo F1 contract — and raced alongside Mika Häkkinen.
In 2024, Kimiläinen expanded into electric boat racing, competing in the UIM E1 World Championship for Team Brady, the team owned by retired American football player Tom Brady. Her teammate was powerboat racer Sam Coleman.
Alongside racing, Kimiläinen has built a parallel career in broadcasting. She is best known in Finland for producing the television programme Finland's Worst Driver for MTV3. She has also appeared on the Finnish editions of Dancing with the Stars and The Masked Singer. As of 2022, she is in a relationship with former Finnish international footballer Sami Hyypiä.