Kovalainen competed in karting from 1991 to 2000, finishing runner-up in the Finnish Formula A championship in 1999 and 2000, and winning the Nordic championship and the Paris-Bercy Elf Masters in 2000. He also finished third in the World Formula Super A championship that year and was elected Finnish Kart Driver of the Year.
He began car racing in 2001 in the British Formula Renault Championship, finishing fourth with two wins and earning Rookie of the Year. He moved to the British Formula Three Championship in 2002 with Fortec Motorsport, using Renault engines as part of his Renault Driver Development backing. Kovalainen was competitive in the second half of the season, taking five wins in the final nine races, and finished third overall behind Robbie Kerr and James Courtney, again earning the Rookie of the Year award.
Kovalainen spent two seasons in the World Series by Nissan, the Renault-backed series above GP2. In 2003 he struggled against experienced teammate Franck Montagny, who won the title. In 2004, moved to Pons Racing, Kovalainen won the championship ahead of Tiago Monteiro with 192 points and six wins. The title earned him the Finnish Driver of the Year award.
Kovalainen entered the inaugural GP2 Series season with Arden International. His campaign was strong across multiple fronts: he won the opening feature race at Imola, dominated the Monaco feature race from pole for 21 laps before a pit stop problem dropped him to fifth, won at the Nürburgring from seventeenth on the grid, and won again at Magny-Cours from fourth. At Istanbul he won the sprint race in wet conditions. At Monza he claimed his second pole position and won the feature race.
Heading into the final rounds Kovalainen led Nico Rosberg by four points. Rosberg and ART Grand Prix found speed in Bahrain that Arden could not match, and Rosberg secured the championship with a feature race victory while Kovalainen finished third. A retirement in the final sprint race left Kovalainen runner-up, fifteen points behind Rosberg at the end of the season.
Renault promoted Kovalainen to a full-time test driver role for 2006, logging over 28,000 km. When Fernando Alonso departed for McLaren, Renault elevated Kovalainen to the race seat for 2007. He scored his first Formula One podium at the 2007 Japanese Grand Prix at Fuji Speedway, finishing second in difficult wet conditions while holding off Kimi Räikkönen in the closing laps.
He moved to McLaren for 2008 alongside Lewis Hamilton. At Silverstone he achieved his first Formula One pole position. At the Hungarian Grand Prix, Kovalainen inherited the race lead when Felipe Massa retired with an engine failure three laps from the end and converted it to win, becoming the 100th driver to win a Formula One Grand Prix. He remained with McLaren for 2009 but the year proved difficult as the MP4-24 was uncompetitive early in the season. He scored 22 points and finished twelfth in the championship.
Kovalainen moved to the newly formed Team Lotus in 2010, where he spent three seasons in uncompetitive machinery, later with the team renamed Caterham F1. Although the cars were unable to score points, Kovalainen was consistently noted for outperforming other drivers in similarly placed teams. He made a brief return in 2013, driving for Lotus F1 at the United States and Brazilian Grands Prix as a short-notice replacement for Kimi Räikkönen, who was recovering from back surgery, finishing fourteenth on both occasions.
In 2015, Kovalainen moved to Japan to compete in Super GT's GT500 class with Team SARD and a Lexus RC F. In 2016, he and co-driver Kohei Hirate won the Super GT championship, with the title decided at the final race of the season at Twin Ring Motegi. It was Kovalainen's first championship since the World Series by Nissan in 2004. He continued in Super GT until the end of the 2021 season, after which he focused on the Japan Rally Championship with Rally Team AICELLO. He won the JN2 class championship in 2021 with six class victories from six rounds.
In November 2023, a routine health check revealed an ascending aortic aneurysm. Kovalainen underwent successful surgery and returned to racing within a few months.
Kovalainen's junior career path — sustained Renault backing, a World Series title, and a closely fought GP2 runner-up season — reflected a well-structured route to Formula One. His GP2 campaign against Rosberg in 2005 was among the tightest inaugural season title fights the series produced. In Formula One, his Hungarian victory gave him a place in the record books and his years with Caterham/Lotus demonstrated professional discipline in difficult circumstances. The Super GT title and subsequent rally success underlined that his racing ability extended well beyond the Formula One paddock.