Kiyonari's introduction to motorsport came through motocross in 1988, an experience he would later describe as not particularly enjoyable. He transitioned to circuit racing in 1996 and made his competitive debut in the All-Japan 125cc Championship in 1998, continuing in that class through 1999. He then contested the All-Japan 250cc Championship across the 2000 and 2001 seasons, achieving a ninth-place finish in the 2000 Suzuka 8 Hours among his notable results. In 2002 he moved to the All-Japan ST600 category and won the championship outright, taking four victories, while also claiming a win in the All-Japan Superbike Championship at TI Aida.
Through 2002 and 2003, Kiyonari served as an official test rider for the Honda Racing Corporation. Following the death of Daijiro Kato at the 2003 Japanese Grand Prix, Gresini Racing required a replacement for the remainder of the season. Team principal Fausto Gresini called Kiyonari, who stepped up to the MotoGP grid from the French round onwards. He collected eight minor points finishes across those races, finishing the season ranked 20th overall with a best result of eleventh.
Kiyonari arrived in the British Superbike Championship in 2004 and finished sixth overall. In 2005 he dominated the early stages of the season, winning the first four races, before sustaining an ankle injury in a heavy crash in round five. He returned for round nine and reclaimed the championship lead from teammate Michael Rutter, but ultimately finished runner-up behind the improving Ducati of Gregorio Lavilla. That year he also made a guest appearance at the Valencia Grand Prix for Sito Pons' Camel Honda team, scoring points.
The 2006 title arrived via a tense finale at Brands Hatch. A double victory at Silverstone in the penultimate round kept him in contention, and although Shane Byrne's heavy crash in the final race forced a stoppage, Kiyonari only needed to finish third. He came home second in the shortened race behind Leon Haslam, winning the championship by eight points.
He defended the title in 2007 with another championship success at Brands Hatch, having posted multiple victories across the season despite early-season pressure from Jonathan Rea and Leon Camier on their factory Hondas.
Kiyonari returned to the BSB in 2010 after a stint in the World Superbike Championship, joining HM Plant Honda alongside Josh Brookes. Double wins at Mallory Park helped propel him to a third British title that season. Further BSB campaigns followed in 2011, 2013, 2014, and beyond, with a runner-up finish in 2014 aboard a BMW S1000RR his best subsequent result.
Kiyonari made his Superbike World Championship debut in 2008 with Ten Kate Honda, partnering Carlos Checa and Kenan Sofuoglu. He came close to a maiden win at Monza's second race, leading into the Parabolica before running wide on the exit and being edged out by Noriyuki Haga and Max Neukirchner, finishing third by just 0.051 seconds. His first World Superbike victories came at Brands Hatch, where he defeated Troy Bayliss in race one and Haga in race two. He also won race two at Donington Park in very wet conditions, demonstrating his ability in difficult circumstances. He finished ninth in the 2008 standings. He remained with Ten Kate Honda for 2009 alongside Checa and Jonathan Rea, finishing eleventh in the championship.
In 2012, Kiyonari headed to Asia with Boon Siew Honda Malaysia Racing in the FIM Asian SuperSports 600cc Championship. He won the SS600 title that season and subsequently emerged as the benchmark rider in Asian Supersport competition.
Kiyonari accumulated four victories at the Suzuka 8 Hours endurance race, in 2005, 2008, 2010, and 2012. His 2008 Suzuka win was achieved alongside former WSBK teammate Carlos Checa, and at the time of active competition he held the record as the rider with most Suzuka 8 Hours wins still racing.
Kiyonari's three British Superbike titles, nearly all achieved with Honda, made him an enormously popular figure among British motorcycle racing fans and a household name in the BSB paddock. His willingness to race across multiple continents and his four Suzuka 8 Hours victories underline a career defined by adaptability and consistent high-level performance across two decades of professional racing.