Karl Felix Helmer Rosenqvist was born on 7 November 1991 in Sweden. He began his career in Formula Renault 2.0 Asia, which he won in 2008, and added the Formula Renault 2.0 Sweden/NEZ title in 2009. A season in the German Formula Three Championship in 2010 produced fifth place overall with two victories, eight podiums, and one pole position, including a seventh-place finish on his debut at the Macau Grand Prix. Those results earned him a promotion to the European championship the following year.
Rosenqvist joined Mücke Motorsport for the 2011 Formula 3 Euro Series, the predecessor to the unified FIA championship. He finished fifth in his rookie year with one win, ten podiums, and five fastest laps — a strong debut that also included victory at the Masters of Formula 3, a prestigious non-championship invitational event. His year-end performance at Macau saw him qualify and compete without a top result.
The 2012 European Formula 3 Championship brought a step forward to third overall in the standings, with a second-place finish at the Macau Grand Prix on the season's non-championship showpiece. Rosenqvist was establishing himself as a consistent front-runner but had not yet converted that consistency into a championship challenge.
The 2013 European Formula 3 Championship was Rosenqvist's closest miss at the title. He narrowly lost to Raffaele Marciello, a result that highlighted the fine margins separating him from championship honours at that stage. Away from the title fight, he won the Masters of Formula 3 for a second time, again from pole position — a demonstration of his ability to deliver when the stakes were highest on a given day. At the Macau Grand Prix, he started from the front row but was eliminated on the first lap after contact with Marciello and Pipo Derani.
Rosenqvist's fourth season with Mücke Motorsport produced a more modest eighth-place finish in the championship, reflecting a year in which the team's competitiveness relative to rivals such as Prema Powerteam declined. However, the season ended with a victory at the Macau Grand Prix from pole position, beating teammate Lucas Auer, which kept Rosenqvist's reputation among the very elite of the category intact.
For 2015, Rosenqvist switched to Prema Powerteam, the outfit that had won the championship the previous year with Esteban Ocon. The move proved decisive. Rosenqvist dominated, winning thirteen races, recording twenty-four podiums, and taking seventeen pole positions across the season to claim the championship by a commanding margin. He also secured his second consecutive Macau Grand Prix victory from pole position that year, giving him back-to-back wins at the event.
The cumulative record across five European Formula 3 seasons — the title, two Macau victories, the Masters of Formula 3 twice — led to Rosenqvist being recognised as the most successful Formula 3 racer in the championship's history up to that point.
Rosenqvist's Formula 3 career ended after 2015 without a conventional GP2 step. He announced a programme in the 2016 Indy Lights series with Belardi Auto Racing, though a clash with European sportscar commitments for Mercedes-Benz limited him to ten of eighteen rounds. He also spent time in DTM as a reserve driver for Mercedes before being promoted to a race seat after Esteban Ocon left for Formula One. He competed in Formula E from 2016 to 2018, winning the 2017 Berlin ePrix for Mahindra, raced in Super Formula and Super GT in Japan, and eventually joined IndyCar full-time with Chip Ganassi Racing in 2019, winning the Rookie of the Year award. He went on to win the 2026 Indianapolis 500 with Meyer Shank Racing.