Nieto built a formidable domestic record before making the step into world-level competition. He won the Spanish 125cc championship in 1998 and followed it with back-to-back Spanish 250cc titles in 1999 and 2000. He also claimed the European 125cc championship in 1997. Alongside those domestic campaigns in 1999 and 2000, he contested the 250cc World Championship on a Yamaha TZ250, finishing 14th overall in 2000 with a best result of sixth at Estoril.
Nieto joined the 250cc World Championship full-time in 2001 and immediately showed his calibre, finishing fourth overall. His campaign was built on consistency — four successive fifth-place finishes early in the season provided a solid base before a pair of late-season podiums confirmed his pace at the front.
The 2002 season was his finest in 250cc. Nieto won four races and finished second in the world championship, held off the title only by the dominant Marco Melandri. He remained a frontrunner in 2003, winning at Donington Park and finishing fifth, and again placed seventh in 2004 before making his exit from the 250cc class.
Nieto entered the Superbike World Championship in 2005 with the Caracchi Ducati team. The transition was gradual — a fourth-place finish in round four stood as the highlight of an otherwise unremarkable debut campaign that ended 17th overall.
He moved to PSG-Kawasaki for 2006 and 2007, where he found more consistent form. His first Superbike podium came with a third place in race two at Assen in 2006, following teammate Chris Walker's victory in race one. He added a further third at Magny-Cours in 2007 and took a notable pole position at Lausitzring that year, having identified a dry window during a wet qualifying session.
Also in 2007, Nieto made his only MotoGP class start, substituting for the injured Olivier Jacque at the French Grand Prix, where he finished eleventh.
The most significant chapter of Nieto's Superbike career came in 2008 with Alstare Suzuki. He made an immediate statement by winning the second race of the opening round in Qatar — his first world superbike victory. He went on to finish seventh in the championship for the season, a career-best Superbike result. Despite the strong campaign, the team scaled back to two riders for 2009 and retained Yukio Kagayama instead. Nieto did return to the team mid-season, however, after Max Neukirchner sustained an injury.
For 2010, Nieto returned to the Grand Prix paddock for the inaugural Moto2 season. Racing under the new technical formula, he suffered a serious crash at Indianapolis that proved career-defining. Unable to fully recover from his injuries, he announced his retirement from competition in early 2011.
Following his retirement, Nieto pursued a parallel career as a DJ and music producer, performing regularly at venues across Spain. He subsequently transitioned into motorsport management, eventually taking on the role of Rider Performance Director at Pramac Racing, the longstanding satellite Ducati MotoGP team, where he applies his competitive experience to the development of the team's riders.
Fonsi Nieto's career encompassed two distinct disciplines at world championship level, with a 250cc runner-up title and a maiden Superbike race win among its highlights. His family connection to Ángel Nieto added a layer of expectation that he ultimately fulfilled with genuine competitive results across two decades of racing.