Marimón was born on December 19, 1923, in Zárate, Argentina. His father, Domingo Marimón, was a racing driver. Juan Manuel Fangio was a close friend of Domingo Marimón and became a mentor to Onofre when he moved to Europe.
Marimón began his racing career in the late 1940s in Argentina, making his international debut in 1951. In June 1951, he debuted at the 24 Hours of Le Mans, driving the #7 Talbot-Lago T26GS for Henri Louveau with co-driver Jose Froilan Gonzalez, but they retired due to a broken radiator. A week later, he made his F1 debut at the French Grand Prix at Reims-Gueux in the #50 Maserati 4CLT for Scuderia Milano, retiring after two laps with an engine issue.
After spending 1952 in South America, Marimón returned to Europe in 1953 as a factory driver for Maserati in the F1 World Championship. He drove a Maserati A6GCM at Spa-Francorchamps in June, finishing third in the Belgian Grand Prix alongside Ferrari drivers Alberto Ascari and Luigi Villoresi. That season, he also finished ninth in the French Grand Prix and recorded four DNFs at Silverstone, Nürburgring Nordschleife, Bremgarten, and Monza. He finished 11th in the 1953 championship standings with four points. Marimón also competed in non-championship F1 races, achieving a second-place finish at the Modena Grand Prix. Outside F1, he participated in sports car races like the Mille Miglia, 24 Hours of Le Mans, and 1000 km of Nürburgring, but recorded DNFs in all of them.
In 1954, Marimón continued with Maserati in Formula One, driving a Maserati 250F. He made eight starts in this car, four in championship events and four in non-championship events. He won the Rome Grand Prix and secured his second F1 championship podium, finishing third at the British Grand Prix at Silverstone. He started 28th and finished 3rd at Silverstone, a record that will never be beaten.
Marimón's career ended prematurely on July 31, 1954, during qualifying practice for the German Grand Prix at the Nürburgring Nordschleife. He was attempting to improve his qualifying time when he crashed. His best time had been the 8th-fastest, 21 seconds behind Juan Manuel Fangio's fastest qualifying time. Marimón was following his mentor Fangio when the accident happened. However, another account states that Marimón was not following Fangio, who had asked him to wait to travel in tandem, but Marimón, due to impatience, went out alone to explore the track, leading to the fatal accident. He became the first fatal victim of F1 Grand Prix races (excluding the Indianapolis 500) since the championship's inauguration.
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