De Simone joined Jordan Grand Prix as a test driver in 1995 and 1996. Jordan, based in Silverstone and founded by Eddie Jordan, was an established midfield Formula One constructor during that period, having scored its first Grand Prix victory at the 1998 Belgian Grand Prix — though De Simone's association with the team predates that breakthrough. Test drivers of the mid-1990s carried out development running on circuits across Europe, providing feedback on setup, tyres, and aerodynamic changes without appearing in race entries. De Simone's two seasons in this role gave him direct experience of Grand Prix-specification machinery during a period when the sport was still transitioning through significant technical regulations changes.
After his time as a Formula One test driver, De Simone moved into front-line touring car competition. Between 1997 and 1999 he raced in the Italian Superturismo Championship, the domestic equivalent of the European Touring Car Cup, which attracted factory-supported entries from Alfa Romeo, BMW, and Renault among others. Three seasons in the series gave him a foundation in production-based racing on Italian circuits.
De Simone extended his touring car career to European level, competing in the European Touring Car Championship in 2000 and 2002. The ETCC attracted manufacturers and professional drivers from across the continent and served as a precursor to the rebranded World Touring Car Championship introduced in 2005.
The final and most extended phase of De Simone's career was devoted to sports car competition. He raced Maserati and Ferrari machines across three of the era's principal GT series: the FIA GT Championship, the American Le Mans Series, and the Le Mans Series. Both the ALMS and Le Mans Series featured GT-class competition at circuits including Road Atlanta, Sebring, Spa-Francorchamps, and Silverstone, and drew Italian marque representation from drivers who could combine pace with mechanical reliability over longer race distances.
Racing Maserati and Ferrari sports cars in these championships placed De Simone among the international pool of Italian competitors who carried those manufacturers in GT competition during the early 2000s, a period that coincided with Maserati's renewed commitment to motorsport following its return to the FIA GT Championship with the MC12.
De Simone's career followed a trajectory common among European racing drivers of his generation: junior categories giving way to a works development role in Formula One, followed by a pivot to the more commercially accessible world of GT and touring car competition. His two seasons with Jordan Grand Prix represent the closest he came to the Formula One grid, while his subsequent sports car programme spanned two continents and three major series.