Kobayashi began karting in 1996 at age nine and spent seven years in the discipline, winning four karting titles including the Toyota SL All Japan Tournament Cadet Class twice. He joined Toyota's Driver Academy in 2004 and progressed through Formula Renault, winning both the Italian and European Formula Renault championships. In 2006 he entered the Formula 3 Euro Series with ASM Formule 3 alongside Sebastian Vettel and Paul di Resta, finishing eighth overall and first in the Rookie championship. A strong run in the Macau Grand Prix and Masters of Formula 3 followed.
After two GP2 Asia Series campaigns, Kobayashi won his first GP2 race from pole at Barcelona in 2008. He made his Formula One debut at the 2009 Brazilian Grand Prix as a last-minute replacement for an injured Timo Glock at Toyota, defending hard against Jenson Button — who was chasing the title — and finishing tenth. A further appearance at Abu Dhabi brought his first championship points. Before Toyota withdrew from F1 at the end of 2009, Kobayashi was poised for a full-time seat.
Kobayashi joined Sauber for the 2010 season alongside Pedro de la Rosa. Despite starting the year with a run of retirements, he showed remarkable overtaking ability and finished the season with 32 points. The 2011 season brought a career-high fifth place at Monaco and a near-podium in Canada. In 2012, driving alongside Sergio Pérez, Kobayashi qualified second at Spa and achieved his maiden Formula One podium with third place at the Japanese Grand Prix at Suzuka — the first Japanese driver to stand on the F1 podium in Japan in 22 years. He finished twelfth in the championship with sixty points before being dropped by Sauber at the end of the season.
A brief return with Caterham in 2014 proved difficult, with the underperforming team unable to provide a competitive platform. Kobayashi did not secure a further F1 seat after that campaign.
Kobayashi joined Toyota Gazoo Racing in the FIA World Endurance Championship as an LMP1 driver in 2016. He scored his first WEC race victory at the 2016 6 Hours of Fuji. In 2017 he set the current outright lap record at Circuit de la Sarthe with a time of 3:14.791. After multiple near-misses at Le Mans, Kobayashi finally won the 24 Hours of Le Mans in 2021 from pole position alongside Mike Conway and José María López. He also won the 2019–20 and 2021 FIA World Endurance Championship driver titles with the same two co-drivers. He won the Daytona 24 Hours in 2019 and 2020 with WTR. A second Le Mans victory followed in 2026, again with Toyota.
In December 2021, Kobayashi succeeded Hisatake Murata as team principal of Toyota's WEC programme while continuing to drive. Under his leadership as team principal, Toyota won three consecutive World Manufacturers' Championship titles from 2022 to 2024.
Kobayashi competed in the NASCAR Cup Series in 2023 and 2024 for 23XI Racing at road-course events, finishing 33rd at Indianapolis in 2023. In June 2025 he tested a Haas VF-23 at Circuit Paul Ricard, returning to a Formula One car for the first time in eleven years courtesy of Toyota's relationship with Haas.
Kobayashi's trajectory — from aggressive F1 overtaker to Toyota's most decorated endurance specialist and its WEC team principal — is one of the more remarkable second-act stories in modern motorsport. His outright Le Mans win in 2021, achieved after years with Toyota during which the car failed at the final lap in 2016, closed one of endurance racing's longest chapters of near-misses. He is the fourth Japanese driver to win at Le Mans.