Toseland was born in Doncaster, South Yorkshire, and raised in Kiveton Park in the Borough of Rotherham. His parents separated when he was young and he was raised by his mother. His stepfather Ken Wright introduced him to motocross bikes and piano at an early age; Toseland began piano lessons at eight and achieved Grade 6. Wright later died by suicide, an event that prompted Toseland to transfer his motorcycle career from off-road to road racing. He was educated at Wales High School in Kiveton Park.
Toseland won the 1995 Junior Road Race Championship before progressing to 125cc racing in the UK Superteen series. He attracted attention by dominating the Honda CB500 Cup in his late teens, which led to a seat with the Castrol Honda World Supersport squad. After two seasons in World Supersport — finishing eighteenth and eleventh — he joined the British Superbike Championship in 2000 with Paul Bird's Vimto-sponsored team, placing twelfth overall.
Toseland joined the GSE team in the Superbike World Championship in 2001 alongside Neil Hodgson. Initially less competitive than his teammate, he developed steadily and by 2003 took his first World Superbike win at Oschersleben, finishing third in the championship overall.
In 2004, following Hodgson and Ruben Xaus's departures to MotoGP, Toseland joined the factory Fila Ducati team as second rider to Regis Laconi. He rode with such consistency that he edged out Laconi at the final races at Magny-Cours to claim the championship by nine points, with a tally of 336 points including three wins and eleven other podiums. At 23 years and 364 days old he was, at the time, the youngest World Superbike Champion in history.
After a difficult 2005 campaign in which Ducati's dominance faded, Toseland switched to the Winston Ten Kate Racing Honda for 2006, winning the season-opener in Qatar and finishing second in the championship behind Troy Bayliss. For 2007 he remained with Ten Kate Honda, opening the season with a first-and-second at Qatar and Philip Island. He won at least one race at each of the first five meetings, built a 66-point lead after taking his first-ever World Superbike double at Brands Hatch, and secured the 2007 championship at the final round at Magny-Cours by just two points over Noriyuki Haga.
On 1 August 2007, Toseland announced he had signed a one-year contract with Tech3 Yamaha for the 2008 MotoGP season, with Colin Edwards as his teammate. Facing eight circuits he had never raced at, he was immediately more competitive than expected, qualifying second and finishing sixth in the opener at Qatar. He finished eleventh overall in the championship with 105 points — a creditable maiden MotoGP season marred by inconsistency.
Toseland stayed at Tech3 for 2009. Tension with Edwards over an engineer swap — which Edwards resented — affected early-season relations, and Toseland received jump-start penalties at Laguna Seca (resulting in disqualification) and Phillip Island. Despite an improved crew chief arrangement, he only finished ahead of Edwards twice during the year. Yamaha confirmed before the Portuguese Grand Prix that Ben Spies would replace Toseland at Tech3 for 2010.
Following his MotoGP exit, Toseland took over Spies's position at the Sterilgarda Yamaha World Superbike team for 2010, with Cal Crutchlow as teammate. He struggled to adapt to the bike early in the season and managed only four podiums before the final two rounds. For 2011 he signed for the factory-backed BMW Motorrad Italia Team aboard the BMW S1000RR. In September 2011, after a wrist injury sustained at an official Aragon testing session in March that required nine surgeries, Toseland officially retired from professional motorsport.
Alongside his racing, Toseland is an accomplished pianist and singer-songwriter. Following retirement he fronted a rock band named Toseland, whose debut single "Life Is Beautiful" premiered in April 2013. The debut album Renegade was released in 2014, with all three lead singles reaching the A-list on national radio stations. A second album, Cradle The Rage, followed in 2016. The band toured supporting Deep Purple and Black Stone Cherry, among others. Toseland later re-recorded the song "We'll Stop At Nothing" as the official anthem for the Special Olympics GB 2017.
He demonstrated his piano playing at the BBC Sports Personality of the Year Awards in 2007, the same year he was nominated for the award and named BBC Yorkshire Sports Personality of the Year. He was awarded an honorary doctorate by Sheffield Hallam University in 2009.
In January 2012 Toseland announced his engagement to singer Katie Melua. They married on 1 September 2012 at the Royal Botanic Gardens in Kew, London. In 2020 Melua revealed that the couple had separated. From 2020, Toseland served as team manager of Wepol Racing, fielding riders in the World Supersport and Supersport 300 classes.
James Toseland is one of only a handful of riders to have won two World Superbike Championships, and one of only three to have won the title for two different manufacturers. His rapid progression from domestic racing to back-to-back WSBK titles, his solid if unspectacular MotoGP seasons, and his unusually public second career in music made him one of the most recognisable British motorsport personalities of the 2000s.