Magnussen started kart racing at the age of 11 and won his first national title the following year. He then won the World Junior Karting title at his first attempt, becoming the youngest man ever to win it, just a few days after his 14th birthday. He repeated the success in 1989 and in 1990 moved to Formula K and won a third World title. He made his single-seater debut in Formula Ford at the Jyllandsring in Denmark and in 1992 he scored seven wins in 19 races in the British FF1600 series, finishing third in the title. He also won the Formula Ford Festival at Brands Hatch. This led to a drive in Formula Vauxhall Lotus and he scored four wins in 1993 before joining Paul Stewart Racing in Formula 3 at the end of the year. In 1994 he dominated the British F3 title with 14 victories in 18 races and after testing a McLaren-Peugeot F1 car he was hired by Mercedes-Benz to race in the German Touring Car Championship in 1995. At the end of the year he made an impressive Formula 1 debut finishing tenth in the Pacific Grand Prix at Aida, subbing for Mika Hakkinen who was recovering from an appendix operation.