Häkkinen began karting at age five when his parents rented a go-kart for him at a local track. He won the Keimola Club Championship in 1978 and 1979, the Finnish Karting Championship in 1981, and three consecutive Formula Nordic 100cc titles from 1983 to 1985. A protege of 1982 World Champion Keke Rosberg, he transitioned to car racing in 1987 and swept the Finnish, Swedish, and Nordic Formula Ford championships in his debut season.
In 1990, Häkkinen won the British Formula Three Championship with West Surrey Racing by nine victories, clinching 121 points. He also won the first heat of the Macau Grand Prix before retiring from the second heat following a last-lap collision with Michael Schumacher, who was classified overall winner.
Häkkinen signed with Lotus for 1991, making his Formula One debut at the United States Grand Prix. A contract dispute during 1992 was resolved in favour of McLaren by the Formula One Contract Recognition Board, and he joined the team as a test driver in 1993. He replaced Michael Andretti for the final three rounds of the season and immediately out-qualified Ayrton Senna at the Portuguese Grand Prix before claiming his first podium — third place — in Japan.
After solid but winless seasons through 1994, 1995, and 1996, Häkkinen's 1995 Australian Grand Prix qualifying session ended in a life-threatening tyre failure at high speed at Brewery corner. He sustained a skull fracture and airway blockage and required an emergency trackside tracheotomy performed by volunteer physician Jerome Cockings from the Royal Adelaide Hospital. He was resuscitated twice before being transported to the hospital, where he remained for approximately two months. His recovery was considered remarkable; he returned to racing in 1996.
Häkkinen took his maiden win at the 1997 European Grand Prix at Jerez after a season in which he led two further races only to retire with engine failures, including at Silverstone where he appeared to be holding off Jacques Villeneuve. He ended 1997 sixth in the championship with 27 points.
The 1998 season opened with controversy when teammate David Coulthard yielded position to allow Häkkinen to win in Australia after Häkkinen had been incorrectly called into the pit lane. He went on to win eight races that year, closing the championship in Japan with 100 points — a comfortable margin over Michael Schumacher — to become the second Finnish driver to win the World Championship.
Häkkinen successfully defended his title in 1999. Schumacher broke his leg at Silverstone, and Eddie Irvine stepped into the championship battle for Ferrari. Häkkinen won five races and clinched the title once more at the Japanese Grand Prix, beating Irvine by two points. The result made him the first driver since Alain Prost in 1985-1986 to win consecutive championships.
The 2000 campaign saw Häkkinen take five wins and lead the championship into the final races, but Schumacher took the title. Häkkinen's most celebrated moment of the year came at the Belgian Grand Prix, where on the Kemmel Straight he simultaneously overtook Schumacher and Ricardo Zonta — passing both cars in a single move across the road — in one of the most discussed passing manoeuvres of the era.
After announcing a sabbatical before the 2001 Italian Grand Prix, citing the desire to spend more time with his family and admitting to a loss of desire following crashes, Häkkinen took his final victory at the United States Grand Prix that year and finished the season fifth in the championship with 37 points. In July 2002 he confirmed full retirement from Formula One.
Häkkinen raced in the Deutsche Tourenwagen Masters for HWA from 2005 to 2007, winning twice at Spa-Francorchamps and at Mugello. He finished fifth in the 2005 DTM standings, his strongest season in the series. He retired from competitive motorsport in November 2007 and moved into driver management, later becoming a McLaren partner ambassador and from 2022 a television analyst for Viaplay alongside David Coulthard.
Michael Schumacher named Häkkinen as the rival from whom he derived the most satisfaction in racing. Adrian Newey, designer of Häkkinen's two championship-winning cars, described the Finn's approach to car development as unconventional but ultimately productive once understood: Häkkinen communicated in sensations rather than technical data, and Newey came to see this as a key advantage in fine-tuning the McLaren MP4/13 and MP4/14.