Born in Roskilde, Magnussen won the 1992 Formula Ford Festival at Brands Hatch, then dominated the 1994 British Formula 3 championship with Paul Stewart Racing, winning 14 of the 18 events and breaking Ayrton Senna's F3 record.

Magnussen scored his first British F3 win that April at Donington Park, also taking pole for the race.

[6] He made his Formula One debut at the 1995 Pacific Grand Prix in Aida, sitting in for Mika Häkkinen, who was unwell; becoming the first Danish to race in the category since Tom Belsø in 1974.

Team founder Jackie Stewart once described him as "the most talented young driver to emerge since Ayrton Senna",[7] but his performance was underwhelming.

He scored his only championship point in his last race in F1, the 1998 Canadian Grand Prix; for the rest of the season he was replaced by Jos Verstappen.

In the Danish Touring Car series he drove a Chevrolet Lacetti for Perfection Racing.

On 11 June 2010, it was announced that Magnussen would make his NASCAR Sprint Cup Series debut driving the No.

[11] On 22 July 2023 he won Aurum 1006 km endurance race in Lithuania, driving a Mercedes-Benz AMG GT3 Evo.

[12] On 16 September 2024 it was announced that Magnussen will take part in the Walter Hayes Trophy at Silverstone in November, driving a GT Motorsport-prepared Van Diemen RF78 in the same Duckhams colours that adorned his car at the 1992 Formula Ford Festival [13] Magnussen's oldest son Kevin is also a racing driver currently competing in the FIA World Endurance Championship and IMSA WeatherTech SportsCar Championship with BMW,[14] having previously competed in Formula One with Haas,[15][16] McLaren[17] and Renault.