Ickx was born in Brussels and was introduced to motorsport by his father, a motoring journalist. He initially competed in motorcycle road racing and trials, winning the European 50cc trials title. After transitioning to touring cars in the mid-1960s and winning the Belgian national saloon car championship in 1965, he attracted the attention of Ken Tyrrell, who entered him in the 1966 German Grand Prix driving a Formula Two Matra. Ickx qualified only behind Denny Hulme and Jim Clark before retiring with suspension failure. He made his Formula One debut in a Cooper at Monza in 1967, finishing sixth despite a puncture on the final lap.
Ferrari signed Ickx for 1968. He took his maiden victory in the rain at the French Grand Prix in Rouen and scored several podiums, finishing fourth in the championship. Moving to Brabham in 1969, he won in Canada and Germany — at the Nurburgring, a circuit where he dominated all season — and finished runner-up to Jackie Stewart by 26 points in what would be one of his most competitive title challenges.
Returning to Ferrari in 1970, Ickx had another chance at the championship. Five wins for him, combined with the death of championship leader Jochen Rindt at Monza, kept the title mathematically open until the penultimate race. Ickx won in Austria, Canada, and Mexico, but Rindt was awarded the posthumous championship after Ickx could not close the points gap with three races remaining. He finished runner-up for the second time.
Subsequent seasons with Ferrari yielded victories at Zandvoort in 1971 and the Nurburgring in 1972 — his eighth and final Grand Prix win — but the team declined in competitiveness by 1973 and he departed mid-season. Stints at Lotus in 1974 and 1975 proved difficult as the team struggled with uncompetitive machinery, and Ickx's final Formula One seasons with Ensign and Ligier saw him adapt poorly to the ground effect era. He retired from Formula One at the end of 1979.
His driving was particularly celebrated in wet conditions and on demanding circuits. He clashed publicly with Stewart over the role of danger in the sport, opposing safety reforms that he felt diminished the challenge. At the 1969 Le Mans start he famously walked slowly across the track, protesting the running start as dangerous; from 1970, all drivers were permitted to start seated with belts fastened.
Ickx's six Le Mans victories earned him the nickname "Monsieur Le Mans," a record that stood until Tom Kristensen surpassed it in 2005. His 1969 win with Jackie Oliver in a Ford GT40 was decided by the smallest winning margin in race history — less than 120 yards — after the Porsche 917s that had been heavily favoured proved unreliable. Three of his Le Mans victories came in partnership with Derek Bell, forming one of the event's legendary driver pairings.
His 1977 win at Le Mans, which he cited as his favourite victory, came after being transferred to the Porsche 936 of Jürgen Barth and Hurley Haywood, which was running in 42nd place. Ickx made up the lost laps to take the lead before a mechanical problem required the mechanics to switch off one cylinder, with the reduced-power car going on to win.
From 1976 onwards Ickx raced Porsche's factory turbocharged prototypes, winning the 935 and 936-era races, and then claiming two World Endurance Championships in 1982 and 1983 with the Porsche 956. He also won the 1977 Bathurst 1000 in Australia alongside Allan Moffat — the last debutant to win the race until 2011 — driving a Ford XC Falcon having had only days of practice in a car he had never previously driven.
Ickx retired from endurance racing in 1985 following the death of Stefan Bellof in a collision at Spa-Francorchamps in which both were involved.
Ickx won the 1983 Paris-Dakar Rally driving a Mercedes-Benz G-Class, demonstrating the breadth of talent that distinguished him from most contemporaries. He was inducted into the International Motorsports Hall of Fame in 2002. In his later years he has served as Clerk of the Course for the Monaco Grand Prix. He was honoured on his 75th birthday in 2019 by Porsche with a special 911 Carrera 4S Belgian Legend Edition painted in X-Blue, referencing his iconic helmet design.