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. In 2004 he joined Toyota's Driver Academy and moved into open-wheel racing via Formula Renault, winning both the Italian and European Formula Renault championships.
In 2006, Kobayashi entered the Formula 3 Euro Series with ASM Formule 3, competing alongside Paul di Resta, Giedo van der Garde, and Sebastian Vettel. He finished eighth in the championship and first in the Rookie standings. He competed at the Macau Grand Prix, qualifying on pole but finishing 19th.
Kobayashi won his first GP2 race in only the second event of the 2008 season at the Circuit de Catalunya following a penalty for Romain Grosjean. He won the GP2 Asia Series title in the 2008–09 campaign, claiming two victories, and had been named as one of Toyota's F1 test drivers from early 2007.
Kobayashi made his Formula One debut at the 2009 Brazilian Grand Prix, deputising for the injured Timo Glock. He qualified eleventh and held off Jenson Button — who needed the result to win the world championship — for several laps before finishing tenth, later promoted to ninth. He scored points again at the Abu Dhabi Grand Prix, finishing sixth.
Kobayashi signed for Sauber for 2010. He quickly gained a reputation as one of the sport's most committed overtakers, with his aggressive braking technique drawing widespread attention. Notable results included seventh place at the 2010 Valencia European Grand Prix, where he passed both Fernando Alonso and Sebastien Buemi on fresh tyres in the closing laps.
In 2011, partnered by Sergio Perez, he scored a fifth place at Monaco and was classified seventh in a rain-affected Canadian Grand Prix after working his way from thirteenth.
In 2012, Kobayashi qualified second for the Belgian Grand Prix and took his maiden Formula One podium with third place at his home Japanese Grand Prix at Suzuka, withstanding race-long pressure from Jenson Button. He became the first Japanese driver to finish on the podium in Japan in 22 years, following Aguri Suzuki at the 1990 Japanese Grand Prix. He finished the 2012 season twelfth in the championship with 60 points.
After failing to secure a 2013 seat, Kobayashi returned to Formula One with Caterham in 2014 alongside Marcus Ericsson. The season was difficult amid the team's financial troubles, and Kobayashi's campaign was interrupted when he was temporarily replaced by Andre Lotterer at the Belgian Grand Prix.
Kobayashi joined Toyota Gazoo Racing's LMP1 effort in 2016 and scored his first WEC victory at that year's 6 Hours of Fuji. In 2017, he set the current outright lap record at the Circuit de la Sarthe with a time of 3:14.791. He won the WEC drivers' championship in the 2019–20 season alongside Mike Conway and Jose Maria Lopez, and repeated the title in 2021 — the same year he won the 24 Hours of Le Mans from pole position with Conway and Lopez. He also won the 24 Hours of Daytona in 2019 and 2020 with WTR.
In December 2021, Toyota appointed Kobayashi as team principal of their WEC programme, succeeding Hisatake Murata. Under his leadership, Toyota won three consecutive World Manufacturers' Championship titles from 2022 to 2024. He combined the management role with continued driving duties.
Kobayashi entered Super Formula in 2015 with Team LeMans, scoring three podiums on the way to fifth in the championship. He moved to KCMG in 2017 and raced with them for eight seasons, winning four second-place finishes during that period, with his WEC commitments occasionally requiring replacements. He added a podium in 2024 at Fuji Speedway before departing KCMG and moving to KDDI TGMGP TGR-DC for a new chapter with rookie Rikuto Kobayashi.
Kobayashi made a one-off debut in the Super GT GT500 class in 2017 with Racing Project Bandoh and raced full-time in the class from 2018 with SARD. He also competed in NASCAR, making his Cup Series debut at Indianapolis in 2023 for 23XI Racing, finishing 33rd, and returned to Circuit of the Americas in 2024. In June 2025, he tested a Haas VF-23 Formula One car at Circuit Paul Ricard, returning to an F1 cockpit for the first time in eleven years.
Kobayashi is the third FIA world champion from Japan and stands as one of the most multifaceted Japanese drivers in the history of international motorsport. His simultaneous career as a Le Mans winner, WEC team principal, Super Formula competitor, and occasional F1 and NASCAR participant spans more categories than almost any of his contemporaries.