Frentzen was born on 18 May 1967 in Mönchengladbach in what was then West Germany. He began karting at age twelve, winning the German Junior Kart Championship in 1981. Moving into car racing in 1985 via Formula Ford, he contested the German Formula Three Championship in 1989 alongside Michael Schumacher and Karl Wendlinger. The three drivers finished on identical points, with Wendlinger taking the title and Frentzen and Schumacher sharing joint runner-up status. The rivalry between Frentzen and Schumacher, which included a disputed incident at Zeltweg where Frentzen alleged he had been forced off the track, would colour their careers for years.
After competing in International Formula 3000 in 1990 and 1991, Frentzen spent three seasons racing in Japan before being signed by Sauber for a Formula One debut at the 1994 Brazilian Grand Prix.
At Sauber, initially powered by Mercedes and later Ford, Frentzen established himself as a consistent points scorer and a driver of considerable natural pace. He took the team's first podium at the 1995 Italian Grand Prix, finishing third. Despite regular points-scoring performances in 1996, the Sauber C15 was not competitive enough for a title challenge. By the end of that year, his move to Williams to replace 1996 champion Damon Hill had been confirmed.
Frentzen's 1997 campaign with Williams alongside Jacques Villeneuve began promisingly. He took the lead at the first corner of the season opener in Australia and ran second before a brake disc exploded. He then won the fourth race of the year at San Marino, his maiden Formula One victory, and set his first career pole position at the following round in Monaco. A run of five consecutive podiums in the second half of the season lifted him to a final championship position of third, subsequently elevated to second following Schumacher's disqualification for his collision with Villeneuve at the European Grand Prix. Despite scoring fewer wins than his team-mate, Frentzen's final tally of 42 points compared to Villeneuve's 81 told the story of a season in which he was generally out-qualified and out-performed by his French-Canadian partner.
The 1998 season was difficult. Williams lost Renault's works engine support and designer Adrian Newey departed to McLaren. Frentzen scored a single podium, at the opening race in Melbourne, and finished seventh in the championship.
Moving to Jordan in a straight swap with Ralf Schumacher, Frentzen enjoyed the most productive season of his career in 1999. Driving the Mugen-Honda powered Jordan 199, he won the French and Italian Grands Prix and accumulated points consistently. At the 1999 European Grand Prix at Nurburgring, he led from pole position and, with the championship leaders Mika Hakkinen and Eddie Irvine both outside the points at that moment, briefly stood on the brink of a three-way tie at the top of the standings — until his Jordan stopped on lap 32 with a mechanical failure.
Before the penultimate race in Malaysia, Frentzen remained mathematically in championship contention but finished sixth while Hakkinen and Irvine scored higher, ending his title hopes. He finished third in the World Drivers' Championship, 22 points behind champion Hakkinen, and was widely regarded as the driver of the year.
The 2000 and 2001 seasons at Jordan proved far more difficult. Honda began supplying the rival BAR team, resulting in Jordan receiving less engine development support. Frentzen managed two podiums in 2000 but the team fell behind BAR in performance. Midway through 2001 Jordan dismissed him — a decision Eddie Jordan later admitted was taken partly to appease Honda and secure the signing of Japanese driver Takuma Sato. Frentzen moved to the struggling Prost team for the remainder of 2001, qualifying as high as fourth at Spa before Prost collapsed financially.
Frentzen signed with Arrows for 2002, scoring points twice and outpacing both Jaguar drivers using the same engine. Arrows went bankrupt in August and released him. He returned to Sauber for 2003, achieving a final podium at the penultimate race in the United States, before retiring from Formula One at the end of the season.
Post-Formula One, Frentzen raced in the Deutsche Tourenwagen Masters from 2004 to 2006 with Opel and then Audi. He also contested the 24 Hours of Le Mans in 2008 with Aston Martin, finishing fourth in class.
Frentzen's career was characterised by flashes of exceptional talent — the 1999 wins and his pace relative to Schumacher in junior formulae being the clearest examples — alongside an inconsistency that prevented him from building on those performances. Team principal Peter Sauber noted that Frentzen worked best in specific atmospheres, and his difficulties at Williams, where he reportedly clashed with technical director Patrick Head, were the most visible illustration of that trait. BBC Sport summarised his career on his retirement as one who "never quite made the most of a brilliant natural talent."