Born in Brussels, Ickx was introduced to motorsport by his father, Jacques Ickx, a motoring journalist. His early competitive career was on two wheels: he won the European 50 cc trials title and several Belgian national titles before moving to touring car racing in the mid-1960s. He won the Belgian national saloon car championship in 1965 and took victory at the 24 Hours of Spa in 1966 in a BMW 2000TI.
His Grand Prix potential attracted the attention of Ken Tyrrell, who entered the 21-year-old in the 1966 German Grand Prix at the Nürburgring in a Formula Two Matra. That debut ended in a first-lap collision with John Taylor, who later died of his injuries. When Ickx returned to the same race in 1967 in another Formula Two car, he qualified third overall against Formula One machinery, overtaking twelve Formula One cars in the opening laps before retiring with suspension failure. That drive earned him a Formula One contract with Cooper, for whom he made his World Championship debut at the 1967 Italian Grand Prix, finishing sixth despite a late puncture.
Ferrari signed Ickx for 1968. He took his maiden win in the French Grand Prix at Rouen in heavy rain — a foretaste of the wet-weather mastery that would become his signature — and finished fourth in the championship. Moving to Brabham in 1969, Ickx won in Canada and at the Nürburgring and ended the year runner-up to Jackie Stewart.
He returned to Ferrari for 1970 and again finished second in the championship, this time behind the posthumously crowned Jochen Rindt. Ickx was the only driver mathematically capable of overtaking Rindt's points total after the Austrian died at Monza, but finishing second in the final race in Mexico left him short. He later stated he was glad not to have won the title against a man who could no longer defend it. Ickx won the Dutch Grand Prix in 1971 — again in the rain, on Firestone wet tyres that Stewart's Goodyear rubber could not match — and claimed his final Formula One victory at the Nürburgring in 1972. After Ferrari's Formula One programme became uncompetitive in 1973, Ickx departed mid-season, making one-off appearances for McLaren, Williams, and Ferrari before eventually joining Lotus in 1974.
The Lotus years were troubled. The team struggled to replace the ageing Lotus 72 with the problematic Lotus 76, and Ickx managed only two third-place finishes of note in 1974. His 1975 season was worse, and he was released partway through the year. After three difficult races with Wolf-Williams in 1976 that included four failures to qualify, Ickx swapped seats with Chris Amon and drove the Ensign N176 for the remainder of the season, showing genuine pace at Zandvoort before an engine failure ended his run. He made scattered appearances for Ensign over the next two seasons before joining Ligier in 1979 as cover for the injured Patrick Depailler. He found the ground-effect cars alien to his precise, mechanical style and retired from Formula One at season's end, having scored eight wins, thirteen pole positions, fourteen fastest laps, and twenty-five podiums across fourteen seasons.
Ickx's endurance record is arguably more remarkable than his Formula One results. He won the 24 Hours of Le Mans six times between 1969 and 1982, a record that stood until Tom Kristensen surpassed it in 2005. The 1969 victory was particularly celebrated: protesting the dangerous Le Mans start, Ickx deliberately walked across the track and was last away, then drove a measured race in an apparently outclassed Ford GT40 to beat the Porsche 908 of Hans Herrmann and Gérard Larrousse by fewer than 120 yards — the smallest competitive margin in the race's history. His advocacy was vindicated: from 1970 all Le Mans drivers were permitted to start seated and belted.
Three of his Le Mans wins came in partnership with Derek Bell, forming one of the race's most celebrated driver pairings. Ickx became a works Porsche driver in 1976 and steered the 936 and later the 956 to multiple victories. He won the World Endurance Championship in both 1982 and 1983. He also won the 12 Hours of Sebring in 1969 and 1972, and the 24 Hours of Daytona in 1972 alongside Mario Andretti, completing the Triple Crown of endurance racing.
In 1977, Ickx co-drove with Allan Moffat to win the Bathurst 1000 in Australia, becoming the last debutant to win the race until 2011. He triumphed in the 1983 Paris-Dakar Rally driving a Mercedes-Benz G-Class, an achievement that underlined his adaptability across entirely different disciplines.
Ickx's career ended on a difficult note. At the 1985 1000 km of Spa, a collision with Stefan Bellof at Eau Rouge resulted in Bellof's fatal crash, an incident that shook Ickx profoundly. He retired from professional circuit racing at the end of that season.
Noted above all for his brilliance in wet conditions and his command of demanding circuits — particularly the Nürburgring, where his head-to-head battles with Jackie Stewart were among the era's defining contests — Ickx was inducted into the International Motorsports Hall of Fame in 2002. He has served as Clerk of the Course for the Monaco Grand Prix and remains an active figure in historic motorsport, typically representing Porsche and Ferrari. A Porsche 911 Carrera 4S special edition named the Belgian Legend Edition was produced in his honour for his 75th birthday in 2019.