Aoyama began racing in MiniMoto at the age of four, competing alongside Yuki Takahashi, a rival he would face across much of his career. He raced in the All-Japan Road Racing Championship for several years, winning the domestic 250cc championship in 2003 with Honda. That same year he made wildcard appearances in the Grand Prix World Championships, finishing second at the Japanese Grand Prix at Suzuka.
Aoyama joined the 250cc World Championship full-time in 2004 with Honda, scoring two third places in his debut season and finishing sixth overall. In 2005 he claimed his maiden victory at his home circuit of Motegi and ended the year fourth in the standings.
For 2006, unable to secure a continued Honda ride, Aoyama switched to KTM. He delivered the manufacturer's first two victories in the class — at Istanbul and Motegi — and again finished fourth overall. He stayed with KTM through 2007 (sixth place, victories in Germany and Malaysia) and 2008 (seventh, with two second-place finishes), even as KTM ultimately withdrew from the class.
With KTM's departure, Aoyama returned to Honda for 2009 with Team Scot, replacing his rival Takahashi who had moved up to MotoGP. The season proved the best of his career: four victories, three second places, and a string of points finishes. At the season finale, Aoyama clinched the 250cc World Championship title, becoming the last rider to win the class in its original form before it became Moto2 the following year. Across six seasons in the 250cc championship, he won nine races and never finished outside the top seven in the overall standings.
Aoyama stepped up to MotoGP in 2010 with Interwetten Racing (as Emmi-Caffè Latte Team Honda), riding the RC212V. The team chose early on not to rely on the machine's electronic rider aids in testing, seeking to understand the bike's fundamentals. His debut MotoGP season was severely disrupted when he sustained a fractured vertebra in a practice crash at Silverstone, sidelining him for the majority of the year.
In 2011 he was more consistent, typically finishing in the lower reaches of the top ten. A notable highlight came when he finished fourth at the Spanish Grand Prix. He also substituted for the injured Dani Pedrosa aboard the factory-spec Repsol Honda at the Dutch TT in Assen — one of the most high-profile stand-in rides in that season.
For 2012, Aoyama moved to the Superbike World Championship, joining Jonathan Rea at Castrol Honda. He competed in MotoGP in various capacities through the 2014 season, after which he retired from professional competition and transitioned to the role of HRC test rider and advisor to young riders in the Shell Advance Asia Talent Cup.
Following his riding career, Aoyama moved into team management. He became team principal of Honda Team Asia, helping develop the next generation of Asian motorcycle racing talent in the Moto2 and Moto3 categories. His younger brother Shuhei Aoyama, who competed in the 250cc class and later World Superbikes, followed a similar career path in professional motorcycle racing.
Hiroshi Aoyama occupies a unique place in motorcycle racing history as the last 250cc World Champion. His title in 2009 closed a chapter that stretched back to the founding of the Grand Prix World Championship, as the 250cc class was replaced by the spec-machinery Moto2 series in 2010. His perseverance through manufacturer changes — cycling from Honda to KTM and back — demonstrated adaptability that few riders in his era matched.