Van Lennep joined the Porsche sportscar team in 1967 and developed into one of the team's trusted endurance drivers over the following decade. His defining moment as a driver came at the 1971 24 Hours of Le Mans, where he shared the number 22 Martini Racing Porsche 917K with Austrian driver Helmut Marko. The pair won the race outright and set a distance record of 5,335 km (3,315 miles) at an average speed that would remain unbeaten until 2010 — the longest-standing race distance record in Le Mans history at the time of its eventual surpassing.
Van Lennep added a second Le Mans victory in 1976, sharing a Porsche 936 Turbo with Jacky Ickx, one of the most prolific Le Mans winners in the history of the race. That partnership confirmed van Lennep's place among the elite endurance drivers of his era.
In 1973, van Lennep won the last running of the Targa Florio — the celebrated open-road race held on the mountain roads of Sicily — sharing the Martini Porsche Carrera RSR with Herbert Müller. The 1973 edition proved to be the final Targa Florio to count as a round of the World Sportscar Championship due to safety concerns, making van Lennep and Müller the final winners of the race in its premier championship incarnation.
Van Lennep's Formula One career was limited but notable. In 1971, the Stichting Autoraces Nederland (Foundation for Car Races in the Netherlands) hired a Surtees TS7 for him to contest his home Grand Prix, the Dutch Grand Prix, where he finished a creditable eighth in a wet race. The following year van Lennep won the 1972 Rothmans European Formula 5000 Championship driving a Surtees TS11 and a McLaren M18.
He made further Formula One appearances for the Williams GP team, earning his first World Championship point with sixth place at the 1973 Dutch Grand Prix. A second championship point followed with Ensign at the 1975 German Grand Prix. His total of two Formula One World Championship points placed him fifth among Dutch Formula One drivers, behind Max Verstappen, Jos Verstappen, Carel Godin de Beaufort, and Christijan Albers at the time of reckoning.
Van Lennep retired from racing following his second Le Mans victory in 1976. His legacy rests chiefly on the 1971 Le Mans record run — a result that combined with Marko's driving to produce what was, at the time, the greatest distance ever covered in the 24-hour race — and on bookending the Porsche 917 era at its peak before the 936 Turbo era took hold. As one of the few Dutch drivers to win at Le Mans and to have competed in Formula One, van Lennep remains a significant figure in Dutch motorsport history.