Haga began racing in the Japanese Superbike Championship in 1993 aboard a Ducati before switching to Yamaha in 1995. He won the Japanese Superbike title with Yamaha in 1997. That same year he teamed with Colin Edwards at the Suzuka 8 Hours endurance race, and the pair won the event — a significant result that drew international attention to the young Japanese rider.
Haga made occasional WSBK appearances from 1994 onward. In 1996 a wild card entry at Sugo saw him finish second in Race 1, his first World Superbike podium at his very first attempt. In 1997, standing in for the injured Colin Edwards, he won his first WSBK race and collected further podiums. When he joined the WSBK full-time in 1998 under the number 41 he would use throughout his career, he began the season by winning three of the first four races.
Haga consistently challenged for the title during the early 2000s. His most painful near-miss came in 2000, when he was a genuine championship contender before testing positive for ephedrine after the South African round. The substance was later identified as naturally occurring in an off-season herbal supplement he had been using. His points from one of the South African races were reinstated on appeal, but a shortened ban still caused him to miss the final round at Brands Hatch. With those losses, Colin Edwards clinched the 2000 title comfortably.
Haga twice left WSBK to compete in the premier class. In the 2001 500cc World Championship he rode the Yamaha YZR500 with Red Bull Yamaha WCM but struggled to adapt, finishing fourteenth without a podium. In 2003 he joined Aprilia's MotoGP programme alongside Colin Edwards on the troubled RS Cube, again finishing fourteenth. Both MotoGP campaigns underlined that his strengths were best suited to the Superbike formula.
Returning to WSBK with Renegade Ducati Koji in 2004, Haga was again in championship contention until the final round. From 2005 he joined the Yamaha Motor Italia factory team, the pairing with which he would produce his finest sustained results.
In 2007, with Troy Corser now as his teammate, Haga and Yamaha won the manufacturer's championship. Haga himself finished runner-up to James Toseland by two points, a double victory at the final Magny-Cours round falling just short of reversing the gap. In 2008 he won seven races, including doubles at the Nurburgring and Vallelunga, yet finished third overall behind Troy Corser and Troy Bayliss.
For 2009 Haga moved to the factory Ducati Xerox team. He won at Phillip Island in the opening round and built an early championship lead, but a severe high-side crash during Race 2 at Donington Park in Round 9 disrupted his title bid and ultimately cost him the championship.
Haga spent 2011 with the Pata Aprilia satellite team before transitioning to the British Superbike Championship, riding for Swan Yamaha in 2012. He finished eighth in his BSB debut year despite nagging injuries. He made further appearances in BSB and the Asia Road Race Series in subsequent seasons before competing in the 2018 CIV Supersport 600 Championship, his final recorded professional campaign.
With 313 career World Superbike race starts — fourth most in the championship's history at the time of his retirement — Haga defined the era of Japanese factory rivalry in WSBK. His three runner-up finishes and 43 victories represent a career achievement that, in any normal era, would likely have included a world title. He remains one of the most beloved figures in Japanese motorsport.