Heidfeld began karting at eleven before winning the German Formula Ford Championship in 1994, taking eight of nine races. He claimed further national championships in 1995 and progressed to the German Formula Three Championship, where he finished third in 1996 and won the title outright in 1997 for Bertram Schäfer Racing, also winning the Monaco F3 support race. In 1998 he was runner-up in the International Formula 3000 championship — losing on the final race when his team used non-compliant fuel, demoting him from pole to ninth — before winning the F3000 crown outright in 1999. He also served as official test driver for McLaren-Mercedes from 1998, and was part of the Mercedes squad at the 1999 24 Hours of Le Mans before the team withdrew after the infamous CLR backflip incidents.
Heidfeld made his Formula One debut with Prost Grand Prix in 2000 alongside Jean Alesi. After a difficult season, he moved to Sauber for 2001, taking his maiden podium with a third place in Brazil while outscoring rookie teammate Kimi Räikkönen. He remained at Sauber for 2002 and 2003, consistently extracting strong results, before a season with Jordan in 2004 and then Williams in 2005 — where he took his sole career pole position at the European Grand Prix before missing two rounds due to injury.
Heidfeld joined the newly formed BMW Sauber team for 2006. He scored the team's first podium at the Hungarian Grand Prix and continued to deliver consistent results over four seasons, reaching a career-best fifth in the World Drivers' Championship in 2007 with 61 points, finishing ahead of teammate Robert Kubica. In 2008 he finished sixth in the championship, completing every race that season — only the second driver to achieve 18 consecutive classified finishes in a single year. When BMW withdrew from Formula One at the end of 2009, Heidfeld spent 2010 as a test and reserve driver for Mercedes and Pirelli before returning mid-season to replace Pedro de la Rosa at Sauber. In 2011 he joined Lotus Renault as a substitute for the injured Kubica, taking a podium in Malaysia before being replaced by Bruno Senna after the Hungarian Grand Prix.
Heidfeld joined the Rebellion Racing LMP1 team from 2012, contesting the FIA World Endurance Championship and the 24 Hours of Le Mans over five seasons from 2012 to 2016. Driving Lola-Toyota and later Oreca-Toyota LMP1 machinery alongside teammates including Neel Jani and Nicolas Prost, he achieved a landmark result at Le Mans in 2012, finishing fourth overall — splitting the Audi factory cars in what was among the strongest privateer performances of that era. In 2014, Heidfeld and the Rebellion crew won the LMP1-L (privateer LMP1) class at Le Mans, and he reached fourth overall on two occasions during his WEC tenure. He also finished runner-up in the 2013 American Le Mans Series standings while racing with Rebellion in North America.
Heidfeld signed for Venturi Grand Prix in the inaugural 2014–15 Formula E season, scoring his first podium at the Moscow ePrix. He joined Mahindra Racing for 2015–16 and remained with the team through 2017–18, adding seven further podiums over three seasons before departing at the end of that campaign.
Heidfeld's career illustrates the gap between consistent high-level performance and the elusive Formula One race win: his 13 podiums without a victory remain the most by any driver in the history of the championship. In a 2016 academic study applying mathematical modelling to assess driver vs. machinery influence, he was ranked 23rd among all-time Formula One drivers. His post-F1 career in endurance racing and Formula E demonstrated that his pace remained competitive well beyond his years in grand prix racing.