Tajima made his racing debut in 1968 in the All Japan Dirt Trial Championship. In 1983 he established Monster Sport International, a motorsport preparation shop. Three years later, in 1986, Tajima formalised his long-standing association with Suzuki by establishing Suzuki Sport, the Japanese manufacturer's in-house motorsport division, before returning to the World Rally Championship. His first WRC appearance had come as early as the 1981 Lombard RAC Rally, where he entered a Datsun. During the late 1980s and into the 1990s Tajima competed in the Asia-Pacific Rally Championship, driving Suzuki models including the Swift, the Baleno Wagon Kit Car, and the Ignis S1600. As the Junior World Rally Championship project accelerated in 2002, Tajima stepped back from rally driving to focus on managing Suzuki Sport.
Suzuki and Tajima began their Pikes Peak challenge in 1989. In 1993, with a twin-engined Suzuki Cultus, Tajima won the Unlimited Division and finished second overall. In 1995 he achieved the event's outright victory using a twin-engined Escudo, becoming the first Japanese driver to win Pikes Peak. In 1996 he began driving the car that defined his Pikes Peak identity: the V6 Suzuki Escudo.
In 2006 Tajima launched what would become a sustained period of dominance. Using a Suzuki Sport XL7 on 21 July 2007, he bettered Rod Millen's thirteen-year-old course record with a time of 10 minutes 1.408 seconds, breaking the 10:05 mark that had stood since 1994. He won outright again in 2008 with a time of 10 minutes 18.250 seconds. In 2009 and 2010, driving a Suzuki SX4, Tajima won again with times of 10 minutes 15.368 seconds and 10 minutes 11.490 seconds respectively. On 26 June 2011, Tajima achieved one of Pikes Peak's milestone moments when he became the first driver to complete the then-partly-unpaved course in under ten minutes, recording a time of 9 minutes 51.278 seconds in the Suzuki SX4 Hill Climb Special developed by Monster Sport. This was his seventh overall victory at the event.
For the 2012 Pikes Peak event Tajima switched to the electric class, citing environmental concerns. His car was the Monster Sport E-RUNNER Pikes Peak Special. Although he led the class in qualifying, a power transfer problem during the race prevented him from completing a competitive run. In 2013 Tajima returned and won the Electric class, placing fifth overall with a time of 9 minutes 46.530 seconds in the E-RUNNER. In 2014 he improved again, setting a new electric class record of 9 minutes 43.90 seconds.
For 2015, Tajima collaborated with Croatian manufacturer Rimac Automobili to produce the Tajima Rimac E-Runner Concept_One. He finished second overall that year with a time of 9 minutes 32.401 seconds, behind Rhys Millen's winning time of 9 minutes 7.222 seconds. In 2016, Tajima again drove the Tajima Rimac E-Runner Concept_One, finishing fifth overall with a time of 9 minutes 51.978 seconds, impacted by brake and overheating issues.
Across his career, Tajima accumulated nine All Japan Dirt Trial Championship titles, four WRC class wins, two class wins in the Asia-Pacific Rally Championship, seven overall Pikes Peak victories between 1995 and 2011, and eight overall Race to the Sky victories in New Zealand from 1998 to 2007. He was inducted into the Pikes Peak Hill Climb Museum Hall of Fame in 2016. Monster Sport's association with Suzuki at Pikes Peak — combining preparation expertise, driver talent, and manufacturer backing — produced one of the longest sustained records of hillclimb success in the event's modern history.