Frentzen began karting at age twelve after his father bought him his first kart, and he showed immediate aptitude. He won the German Junior Kart Championship in 1981 at the age of fourteen. After progressing through Formula Ford, he reached the German Formula Three Championship in 1989, competing directly against Michael Schumacher and Karl Wendlinger. Both Frentzen and Schumacher finished as joint runners-up to Wendlinger on identical points totals, in a season heavily watched by German motorsport authorities seeking to promote a German driver to Formula One.
Frentzen spent three seasons racing in Japan before signing with Sauber for 1994, where he partnered fellow Mercedes junior Wendlinger.
Frentzen made his Formula One debut at the 1994 Brazilian Grand Prix. He stepped into a team leadership role mid-season when Wendlinger was seriously injured in Monaco qualifying. Frentzen showed measured pace and consistency across three seasons with the Swiss team, taking Sauber's first podium at the 1995 Italian Grand Prix, where he finished third. His performances were sufficient to attract the attention of Williams, then the dominant team in Formula One.
For 1997, Frentzen replaced reigning World Champion Damon Hill at Williams alongside Jacques Villeneuve. He took his maiden victory at the San Marino Grand Prix and claimed eight front-row starts through the season, ultimately finishing runner-up after Michael Schumacher was disqualified from the standings following a collision with Villeneuve at the season's final round. Despite this impressive headline result, Frentzen was generally out-performed by Villeneuve, out-qualifying him only four times across the year.
The 1998 season was more difficult. Williams lost both Renault engine support and chief designer Adrian Newey to McLaren, and the team's competitive edge evaporated. Frentzen managed just a single podium and finished seventh in the championship. His difficulties adjusting to the internal culture at Williams — particularly disagreements with technical director Patrick Head — contributed to a mutual parting, and he swapped seats with Ralf Schumacher to join Jordan.
The move to Jordan proved transformative. In 1999, driving the Mugen-Honda powered Jordan 199, Frentzen won at the French and Italian Grands Prix and finished third in the World Drivers' Championship behind Mika Häkkinen and Eddie Irvine, just 22 points from the title. Many in the paddock regarded him as the driver of the year. He came closest to the title at the European Grand Prix, where he led and was poised to draw level on points with both Häkkinen and Irvine before his car stopped on lap 32.
The 2000 and 2001 seasons were more troubled. Honda began supplying BAR as well, creating internal competition for resources, and Jordan's form dropped. Mid-way through 2001, Jordan terminated Frentzen's contract — a decision Eddie Jordan later revealed was partly motivated by pressure from Honda to sign Japanese driver Takuma Sato — and replaced him with Jean Alesi.
Frentzen took Alesi's place at the struggling Prost team for the remainder of 2001, qualifying an encouraging fourth at Spa before the team went bankrupt at year's end. He spent 2002 with Arrows, scoring twice before the team also collapsed mid-season. A return to Sauber for 2003 gave him one final podium at the United States Grand Prix before he retired at the season's close.
In total, Frentzen scored three victories, two pole positions, six fastest laps, and 18 podiums across his Formula One career.
Frentzen moved to the Deutsche Tourenwagen Masters from 2004 to 2006, racing initially for Opel and then Audi. He achieved several podium finishes in the series. In 2008 he competed at the 24 Hours of Le Mans for Aston Martin Racing alongside Karl Wendlinger and Andrea Piccini, finishing fourth in class in the GT1 category.
Frentzen is remembered as one of the most naturally gifted drivers of his generation, often spoken of alongside Michael Schumacher from their shared Formula Three rivalry. BBC Sport described him on his retirement as a driver who "never quite made the most of a brilliant natural talent." His best spell — the 1999 Jordan season — showed what he was capable of given the right environment, but finding that environment consistently proved elusive. Team principal Peter Sauber, who worked with Frentzen at both the beginning and end of his career, observed that the German needed a specific atmosphere to perform, and that conflicts with Williams' Patrick Head had cost him dearly in what should have been his peak years.