Modena started out in karting and was already being tipped as a future star. When he moved up to Formula 3 he was immediately on the pace, finishing fourth in the Italian championship in 1986, and doing himself no harm by taking a superb second place in the Monaco Grand Prix support race and then earning the title of 'European Champion' after winning a one-off race at Imola.
The next year Modena moved up to F3000. Driving for the Onyx team and he took the championship at the first attempt, winning three races (Vallelunga, Birmingham and Imola). At the end of the year he made his Formula 1 debut at the season-ending Australian Grand Prix for Brabham BMW.
For 1988, Modena signed up for the new EuroBrun team, but found the car uncompetitive. He qualified for 12 out of the 16 races, with a best result of 11th at the Hungarian Grand Prix.
In 1989 he returned to Brabham where he regularly matched his more experienced teammate Martin Brundle, scoring points at the Monaco Grand Prix, where he finished 3rd.
Modena remained at Brabham for 1990, but the team were low on funds and apart from a 5th place at the first Grand Prix of the year in the United States, the year was something of a disappointment.
In 1991 he signed for the resurgent Tyrrell team, replacing Jean Alesi.
The year started well with a 4th at the United States Grand Prix and then running 3rd at San Marino before his engine failed. Then at Monaco he was close behind Ayrton Senna when he suffered another engine failure. In Canada he finishing 2nd, however that was about it and as the season went on the team dropped further behind. Frustrated by the team’s performance, Modena retired into his shell.
In 1992 he signed to drive the Jordan-Yamaha. After his experience with Tyrrell it was surprising to many that he would join the Irish team who were running underpowered and unreliable Yamaha engines. Predictably Modena struggled all year, failing to qualify four times. He was unpopular with the team who felt that his unsympathetic treatment of the machinery was part of the problem, and once again his introspective personality saw him retire into his shell. He scored a point in the last race of the year for 6th place at the Australian Grand Prix (his last GP appearance), but was without a seat in 1993, instead entering the Superturismo Italian touring car championship with a BMW.
In 1994 he stayed with touring cars and his excellent early season form in the Alfa Romeo T155 saw him move to the higher-profile DTM late on in the season. He made a sensational debut at AVUS, qualifying third and winning both heats, thoroughly eclipsing the established Alfa aces.
Stefano was signed to race for a full DTM/ITC season in 1995 with hopes high for even greater success. Unfortunately Alfa were lost in technical mire, and could not compete with Mercedes and Opel.
Subsequently Modena, who had harboured hopes of gaining a ride in CART, has been a mainstay of the German Super Touring championship. The 1997 season with the ageing Alfa 155 was a disaster and the arrival of the 156 model the following year rarely allowed him to finish in the top ten, but 1999 saw an upturn in the Italian's fortunes as the 156 at last became a competitive proposition.
In his Grand prix career he participated in 81 grands prix, debuting on November 15, 1987. He achieved 2 podiums, and scored a total of 17 championship points.