Haslam was exposed to motorcycle racing from infancy, regularly accompanying his father Ron Haslam to race meetings. He became the National Youth Motocross champion in both 1995 and 1996, and was named MCN Young Rider of the Year in 1998 after finishing seventh in the British 125cc Championship.
He entered international Grand Prix racing from 2000, spending a season with the Italjet team in the 125cc World Championship and then riding a privateer Honda NSR500V in 2001, becoming the youngest rider ever to compete in the series. Further GP appearances on a 250cc Honda in 2002 added to his experience, though competitive machinery was never available to him at this stage.
Haslam returned to Britain for 2003, signing with Renegade Ducati initially for the British Supersport Championship before being promoted to the superbike class. He also made six World Superbike appearances that year, recording a best finish of sixth at Assen and Magny-Cours.
For 2004, Haslam completed a full World Superbike season alongside Noriyuki Haga with Renegade Ducati, finishing eighth overall with a best result of third and earning Rookie of the Year honors. He also won a British Superbike race at Brands Hatch in torrential rain — the only Ducati victory in the BSB series that year.
Between 2005 and 2008, Haslam was a consistent front-runner in the British Superbike Championship with the GSE Airwaves Ducati team. He finished fourth overall in 2005 and challenged strongly for the 2006 title, accumulating a series of second-place finishes before ultimately ending the season second overall. He joined HM Plant Honda for 2008, winning at Knockhill and completing a double at Cadwell Park.
After seasons competing in World Superbikes with various manufacturers, Haslam returned to British Superbikes between 2016 and 2018 aboard Kawasaki machinery. He won the 2018 British Superbike Championship at the final round at Brands Hatch, securing the title before moving to the Kawasaki factory team in the World Championship alongside Jonathan Rea for 2019.
Haslam's return to World Superbikes in 2009 with the Stiggy Honda team produced a strong debut, including a third place at Phillip Island and back-to-back podiums at Assen, with him finishing sixth overall as the top privateer.
He then signed with Alstare Suzuki for 2010, winning his first World Superbike race at the Phillip Island opener by 0.004 seconds from Michel Fabrizio — the closest finish ever recorded in a World Superbike race at that time. Haslam mounted a serious title challenge that season before mechanical failures and fortune intervened, with Max Biaggi ultimately claiming the championship.
After stints with BMW from 2011 and Ten Kate Honda from 2013, Haslam enjoyed notable success in endurance racing during this period. In July 2013, he won the Suzuka 8 Hours partnered with Michael van der Mark and Takumi Takahashi, then repeated the victory in the 2014 Suzuka 8 Hours with the same team.
After years with various manufacturers including Aprilia, Haslam joined the Kawasaki Racing Team factory squad for 2019. He moved to the HRC-run Honda factory team for 2020 and 2021, competing on the then-new CBR1000RR-R under full factory Honda control. From 2022, Haslam returned to British Superbikes with Lee Hardy Racing on a Kawasaki ZX-10RR.
Leon Haslam represents one of the most experienced figures in World Superbike competition, having ridden for factory-level Honda, Kawasaki, Suzuki, BMW, and Aprilia teams across his career. The Suzuka 8 Hours victories in consecutive years and the historic closest-margin win in World Superbike history stand among his most memorable achievements. As the son of Ron Haslam, he also exemplifies one of British motorcycle racing's most recognizable racing families.