Itoh competed in the All-Japan 500cc Championship on a Honda NSR500 from 1988 to 1992, winning the domestic title in 1990 and consistently finishing in the top seven. During this period he also made occasional wildcard appearances in the 500cc World Championship, scoring a best finish of fourth at Suzuka in 1992. These performances demonstrated a natural ability to run competitively against established Grand Prix riders from the outset.
Honda rewarded Itoh with a full-time 500cc World Championship ride in 1993, placing him alongside Mick Doohan and Daryl Beattie as a third factory entry. In that debut season he scored four top-five finishes and claimed pole position at Hockenheim, where he also finished third. He also made history by becoming the first Grand Prix rider to break the 200 mph (321.86 km/h) barrier in timed practice. He finished seventh in the championship.
In 1994, Itoh scored points in eleven of fourteen races, with nine top-five finishes. His best result came at Brno, where he finished second just three seconds behind Doohan. He again ended the season seventh overall.
The 1995 season brought a moment of near-tragedy. Leading his home race at Suzuka in torrential rain with seven laps remaining, Itoh was caught out by the conditions and crashed while holding a commanding advantage. Despite that setback, he finished fifth in the championship — his best Grand Prix season — courtesy of consistent scoring and two podium finishes.
In 1996, Honda moved Itoh onto the new V-twin NSR500V as a development platform. The bike was underpowered relative to the V4, and Itoh's best result was sixth. He scored points in twelve of fifteen races and finished twelfth overall, concluding his final full Grand Prix season.
Itoh's record at the Suzuka 8 Hours is exceptional. He took his debut win at the event in 1997, partnered by Tohru Ukawa, and the pair repeated from pole position in 1998. A third victory followed in 2006 alongside Takeshi Tsujimura. A fourth win came in 2011 when Itoh came out of retirement specifically for the race. His five pole positions at Suzuka equalled the record held by Wayne Gardner.
He won the Japanese Superbike Championship in 1998, 2005, and 2006, demonstrating sustained competitiveness at domestic level across more than a decade.
In 2005, Ducati brought Itoh into a Bridgestone tyre test team for MotoGP development, making him the first Japanese rider to pilot a Ducati Desmosedici in a Grand Prix when he stepped in for the injured Loris Capirossi at the Turkish Grand Prix. He was excluded from the results after failing to comply with a ride-through penalty. In 2000, he had worked alongside Nobuatsu Aoki testing Bridgestone tyres on NSR500 machines for Kanemoto Racing.
Itoh received a Honda RC211V for the inaugural MotoGP round at Suzuka in 2002, qualifying third — just 0.2 seconds behind Valentino Rossi on pole — and finishing fourth. He returned to Grand Prix action at Motegi in 2007 on a Pramac d'Antin Ducati and again in 2011 as a wildcard at Motegi, where he and Kousuke Akiyoshi were entered to express solidarity with the region affected by the Tohoku earthquake and tsunami. Itoh was 44 years old at that final Grand Prix appearance.
Shinichi Itoh's career illustrates how a rider can sustain relevance across more than two decades through technical excellence rather than headline results alone. His reputation as a development rider of the highest order was built on an ability to provide precise feedback and extract competitive performance from machinery in its formative stages, qualities that kept Honda and Ducati returning to him long after his competitive prime.