Nakajima is the son of the retired Formula One driver Satoru Nakajima.
His father had been backed by Toyota's arch-rivals Honda through his career. Nakajima hoped that by joining Toyota he would shield himself against any accusations that his father had promoted his career.
In 2002, Nakajima won a scholarship in Formula Toyota, which he became champion in a year later. He progressed onto Japanese Formula Three in 2004, winning two of the 20 races and finishing fifth in the Drivers' Championship.
Nakajima stayed in Japanese Formula Three for 2005, finishing second. He dovetailed that championship with appearances in the Japanese GT300 sports car series, where he ended the year eighth.
Nakajima moved to the Formula Three Euroseries in 2006 and competed against the likes of Sebastian Vettel and Paul di Resta. After starting the year strongly with second place in the first race and a win in round four, Nakajima finished seventh with 36 points, behind his Manor Motorsport team-mates Kohei Hirate (third) and Esteban Guerrieri (fourth). The championship was won by di Resta with 86 points.
In November 2006, Nakajima was named a Williams test driver for the 2007 season, alongside fellow test driver Narain Karthikeyan and race drivers Nico Rosberg and Alexander Wurz, targeting a race seat in 2008. Nakajima's debut in a Formula One car came at Fuji Speedway during November 2006, where he completed four laps in wet conditions.
Nakajima raced in the GP2 series in 2007 for the DAMS team alongside French 2005-06 A1 Grand Prix winner Nicolas Lapierre. Nakajima also served as Williams' test driver, completing 7,000km of testing for the team.
Nakajima's first year in GP2 finished with no wins, but five consecutive podiums and ended the year as top rookie. Nakajima's fifth in the championship put him comfortably ahead of Lapierre. Nakajima was found to have caused a collision in Istanbul, when he hit leader Karun Chandhok during the sprint race, and was given a drive-through penalty.
It was announced on 9 October 2007 that following the retirement of Alexander Wurz, Nakajima would race for Williams in the season finale in Brazil. Nakajima finished tenth in the race, setting the fifth fastest lap – quicker than his team-mate Nico Rosberg, who finished fourth.
At his first pit stop, Nakajima overshot his box and hit two of his mechanics. The mechanics were taken to hospital for precautionary checks. Nakajima apologised for the error: “First of all I would say I'm really sorry that some of my mechanics were injured during my pitstop and that I hope they're OK. It was a good first race for me but it was slightly overshadowed.”
Patrick Head commented: “Kazuki drove well on his debut. His lap times were impressive and he's set a marker for a future in Formula One. Some of our mechanics were injured today, they're having some checks done now and we send our best wishes to them.”
On 7 November it was confirmed by Williams that Nakajima would partner Rosberg at the Williams team for the 2008 season. He had a successful start to 2008 at the Australian Grand Prix, finishing 7th but promoted to 6th after Rubens Barrichello was disqualified. He then finished 7th in the Spanish Grand Prix, having outqualified his teammate. A first-corner incident with Giancarlo Fisichella at Istanbul forced him to retire. Nakajima scored two points at Monaco where no Japanese Formula One driver had previously scored a point, and retired from the 2008 Canadian Grand Prix after hitting the pit wall when pitting for a new front wing. Kazuki scored another point at the 2008 British Grand Prix. In Singapore, Kazuki made it to the 3rd qualifying round for the first time qualifying 10th on the grid. He went on the finish 8th and score a point.
| Season | Series | Team Name | Races | Poles | Wins | Points | Final Placing |
|---|---|---|---|---|---|---|---|
| 2003 | Formula Toyota | ? | ? | ? | ? | ? | 1st |
| 2004 | All-Japan Formula Three | TOM'S | 20 | 2 | 2 | 138 | 5th |
| Macau Grand Prix | TOM'S | 1 | 0 | 0 | N/A | 13th | |
| Bahrain F3 Superprix | TOM'S | 1 | 0 | 0 | N/A | 7th | |
| 2005 | All-Japan Formula Three | TOM'S | 20 | 3 | 2 | 209 | 2nd |
| Macau Grand Prix | TOM'S | 1 | 0 | 0 | 0 | 5th | |
| Super GT (GT300) | Kicchouhouzan with apr | 7 | 0 | 1 | 52 | 8th | |
| 2006 | Formula Three Euroseries | Manor Motorsport | 20 | 0 | 1 | 36 | 7th |
| Macau Grand Prix | Manor Motorsport | 1 | 0 | 0 | N/A | NC | |
| Masters of Formula Three | Manor Motorsport | 1 | 0 | 0 | N/A | 26th | |
| 2007 | Formula One | Williams | 1 | 0 | 0 | 0 | 22nd |
| GP2 Series | DAMS | 21 | 1 | 0 | 44 | 6th | |
| 2008 | Formula One | Williams | 15 | 0 | 0 | 9* | 15th* |
| Year | Entrant | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | 19 | 20 | 21 | DC | Points |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2007 | DAMS | BHR FEA 17 | BHR SPR 6 | ESP FEA 15 | ESP SPR 7 | MON FEA 10 | FRA FEA 17 | FRA SPR 6 | GBR FEA 3 | GBR SPR 3 | EUR FEA 3 | EUR SPR 3 | HUN FEA 2 | HUN SPR Ret | TUR FEA 6 | TUR SPR Ret | ITA FEA DSQ | ITA SPR 18 | BEL FEA Ret | BEL SPR 9 | VAL FEA 3 | VAL SPR 7 | 6th | 42 |
| Year | Entrant | Chassis | Engine | 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14 | 15 | 16 | 17 | 18 | WDC | Points |
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 2007 | AT&T Williams | Williams FW29 | Toyota RVX-07 2.4 V8 | AUS TD | MAL TD | BHR | ESP | MON | CAN TD | USA TD | FRA | GBR | EUR | HUN | TUR | ITA | BEL | JPN | CHN TD | BRA 10 | 22nd | 0 | |
| 2008 | AT&T Williams | Williams FW30 | Toyota RVX-08 2.4 V8 | AUS 6 | MAL 17 | BHR 14 | ESP 7 | TUR Ret | MON 7 | CAN Ret | FRA 15 | GBR 8 | GER 14 | HUN 13 | EUR 15 | BEL 14 | ITA 12 | SIN 8 | CHN | JPN | BRA | 15th* | 9* |