Born in Rambouillet, Île-de-France, France, Robert Benoist's father worked for Baron Henri de Rothschild, a member of the wealthy Rothschild family. As a young man, Benoist served during World War I in the French infantry and then for a short time as a fighter pilot in the new Armée de l'Air. At the war's end he started racing automobiles, competing in rallies and road races, and becoming a works driver for Delage in 1924. The next year, teamed with Albert Divo, he won the French Grand Prix in the race that claimed the life of Italian racing star Antonio Ascari.
In 1927, driving a Delage 15-S-8, he won the French, Spanish, Italian and British Grand Prix races, earning the season championship title for the French manufacturer. He is the only driver ever to win these major Grand Prix races in the same year and his accomplishment earned him the Legion of Honor from the French government.
When the Delage company dropped out of racing, Robert Benoist joined the Bugatti team, finishing second in the 1928 San Sebastian Grand Prix in Spain. The following year he teamed up with Attilio Marinoni to win the SPA 24 Hours race in Belgium, driving an Alfa Romeo. At the end of the season he retired until 1934, when he made a comeback with the Bugatti team. He raced with reasonable success, winning the 1935 Grand Prix de Picardie and the Course de Côte de Château-Thierry driving a Bugatti T53. In 1937 he partnered with Jean-Pierre Wimille to win the 24 hours of Le Mans endurance race. Following that victory, Benoist retired permanently, spending most of his time at his country home in Auffargis in the Yvelines département not far from Paris.
In addition to Jean-Pierre Wimille, Robert Benoist became good friends with another Grand Prix driver, William Grover-Williams. When World War II broke out and the Nazis occupied France, these three race drivers all escaped to England where they joined the Special Operations Executive as secret agents to return to France to assist the French Resistance. Parachuted into France, Benoist helped organize sabotage cells and with William Grover-Williams moved weapons from air-drops in the Rambouillet forest to his home at Auffargis for storage and distribution.
In June of 1943, the "Prosper" network in Paris collapsed as a result of an informant and its leaders, Francis Suttill and Andrée Borrel, were arrested by the Gestapo. In August, Benoist's home was raided by the Gestapo and Grover-Williams was captured and executed at the Sachsenhausen concentration camp. Three days later, Robert Benoist was apprehended in Paris. While being driven to Gestapo headquarters, Benoist leaped from the moving vehicle and escaped, eventually being smuggled back to England via the underground. Benoist would later return to France on a second mission, lasting from October 1943 to February 1944, after which he returned to London for a short time before going back to France in March to work in the Nantes area with fellow SOE agent Denise Bloch.
Following Germany's surrender, on September 9 [ (the date of the first anniversary of his death) the "Coupe Robert Benoist" automobile race was held in Paris in his memory.
Captain Robert Benoist is recorded on the Brookwood Memorial in Surrey, England and as one of the SOE agents who died for the liberation of France, he is listed on the "Roll of Honor" on the Valençay SOE Memorial in the town of Valençay, in the Indre departément of France.
In his honor, the village of Auffargis named a street after him and it is there in the churchyard cemetery on "Allée Robert Benoist" that fellow pioneer race driver, Ferenc Szisz is buried.