Ickx was introduced to motorsport through his father, motoring journalist Jacques Ickx. He began competing in motorcycle road racing and trials in the early 1960s, winning the European 50 cc trials title and defeating future motocross world champion Roger De Coster in the 1963 Belgian national 50 cc trials championship. He transitioned to circuit racing in touring cars, winning the Belgian national saloon car championship in 1965 driving a Lotus Cortina and the 24 Hours of Spa in 1966 in a BMW 2000TI.
Ickx entered Formula One for the first time at the 1966 German Grand Prix, driving a Formula Two Matra for Ken Tyrrell. He made his full Formula One debut at Monza in 1967 for Cooper, finishing sixth despite suffering a puncture on the final lap. His speed in F2 machinery at the Nürburgring in 1967 — where he qualified faster than all but two of the Formula One field despite starting behind all of them — attracted widespread attention.
Ickx joined Ferrari for the 1968 Formula One season, winning his maiden Grand Prix in the rain at Rouen in France. He signed for Brabham in 1969 and finished runner-up to Jackie Stewart in the Drivers' Championship despite Stewart's dominant campaign. Returning to Ferrari in 1970, Ickx again finished runner-up, this time to the posthumously crowned Jochen Rindt, winning at Austria, Canada, and Mexico. His relationship with the Nürburgring was exceptional: from 1968 through 1972 he and Stewart shared victories at the circuit, and Ickx won his final Formula One race there in 1972. He later stated he had no desire to win the 1970 championship against a driver who could not defend it.
After leaving Ferrari mid-season in 1973, Ickx drove for McLaren, Williams, and Lotus before eventually joining Ensign in 1976. His final Formula One appearances came with Ligier in 1979, where he substituted for the injured Patrick Depailler. The ground-effect era's aerodynamic demands were ill-suited to his precise, mechanical driving style, and he retired from Formula One at the season's end with eight wins, 13 pole positions, 14 fastest laps, and 25 podiums across 116 Grands Prix.
It was at Le Mans and in endurance racing that Ickx built his most enduring legacy. His 1969 Le Mans victory with Jackie Oliver in a Ford GT40 for John Wyer's team became one of motor racing's defining moments. Ickx opposed the traditional running Le Mans start on safety grounds and walked slowly to his car while rivals sprinted; private driver John Woolfe, who had not buckled his belt, died in a crash on the first lap. In the final hours, Ickx and the Porsche 908 of Hans Herrmann dueled continuously, changing places on each lap; Ickx won by less than 120 yards — the smallest competitive margin in Le Mans history to that point. From 1970, all Le Mans drivers were permitted to start seated with their belts fastened.
Ickx won Le Mans a further five times: 1975, 1977, 1981, 1982 (with the new Porsche 956), and 1985, becoming universally known as Monsieur Le Mans. Three of his victories were alongside Derek Bell. He also won the 12 Hours of Sebring in 1969 and 1972, and the 24 Hours of Daytona in 1972 with Mario Andretti, completing the informal Triple Crown of endurance racing. Ickx won the World Endurance Championship in 1982 and 1983 driving Porsches, earning a second world title alongside his Formula One runner-up positions.
A later episode underlined the personal cost of his endurance career. In 1985 at Spa-Francorchamps, Ickx's car and that of young German Stefan Bellof collided at Eau Rouge while they disputed the lead; Bellof died of his injuries, and Ickx retired from professional circuit racing at the end of the season.
Between 1981 and 2000, Ickx entered 14 editions of the Dakar Rally. He won the 1983 edition in a Mercedes-Benz G-Class, adding a major rally title to a career already spanning almost every branch of motorsport. He also won the 1977 Bathurst 1000 in Australia alongside Allan Moffat in a Ford Falcon, and took the Can-Am title in 1979.
Ickx was inducted into the International Motorsports Hall of Fame in 2002 and the Motorsports Hall of Fame of America in 2020. Porsche issued a special 911 Carrera 4S Belgian Legend Edition in honour of his 75th birthday in 2019. He has served as Clerk of the Course for the Monaco Grand Prix. Particularly celebrated for his mastery of wet conditions and demanding circuits, Ickx is considered by many analysts the most complete racing driver of his generation.